roughly 
 2.7 billion people depend on the burning of biomass and 
 other solid fuels in three stone fires ( tsf ) and other traditional 
 cookstoves for their day - to - day cooking purposes .
cookstoves have environmental and health impacts on an 
 enormous scale , due in large part to emissions 
 of products of incomplete combustion ( pic ) such as co , pm2.5 , methane ( ch4 ) , and polycyclic aromatic hydrocarbons 
 ( pahs ) .
black carbon ( bc ) , commonly known as soot , is an aerosol component 
 formed during combustion that is estimated to have the second highest 
 global warming impact after co2 .
approximately 
 25% of global annual bc emissions and 6080% of africa s 
 and asia s bc emissions that are not from open burning ( e.g. , 
 wildfires ) are from domestic solid fuel combustion .
bc is coemitted with organic carbon ( oc ) , a component which 
 is often regarded to have a cooling impact on climate , although recent studies suggest that the oc fraction ( generally 
 called brown carbon or brc ) that absorbs radiation at short wavelengths 
 contributes significantly to warming .
the net climate impacts 
 of cooking - related aerosol emissions are uncertain , though likely 
 warming .
replacement of traditional cookstoves with alternative technologies 
 thus has the potential to provide considerable climate and health 
 benefits by reducing emissions and human exposures .
efforts to reduce these impacts have spurred the development 
 of 
 a range of alternative cookstoves with varying configurations , levels 
 of sophistication , and performance .
models range from rudimentary 
 low - cost cookstoves often built from local materials to mass produced 
 state - of - the - art forced draft cookstoves ( fdcs ) which use electrically 
 driven fans for improved combustion efficiency . using laboratory emission 
 factors ( ef ; g ( kg wood ) ) , grieshop et al .
estimated that health ( quantified as daily intake 
 of pm2.5 for users ) and climate impacts ( quantified as 
 global warming commitment or gwc ) of various cookstove - fuel combinations 
 can each span 2 orders of magnitude , with all biomass - burning cookstoves 
 having greater impacts than  modern  fuel stoves such 
 as lpg and kerosene .
this variation across cookstoves types 
 and the need to benchmark 
 performance has led to the development of a tier framework , with emissions and other parameters quantified 
 during standardized laboratory testing ( e.g. , the water boiling test ; 
 wbt ) . while laboratory testing is required 
 for benchmarking , field data indicate that laboratory tests typically 
 greatly overestimate performance relative to in - home use .
for example , 
 field pm efs are 25 times higher than those measured during 
 wbt tests .
this is important because benefit estimates for alternatives 
 rely on accurate estimates of real - world performance .
fdcss have the potential to greatly reduce emissions via improved 
 combustion efficiency ; their laboratory pm2.5 and elemental 
 carbon ( ec ) efs are an order of magnitude lower than those for traditional 
 stoves , putting fdcss in the highest tiers ( 3 or 
 4 ) for indoor pm emissions .
however , in - field measurements of emissions 
 from fdcs are limited , report real - time bc concentrations ( not efs ) , and neglect other species ( e.g. , co2 , oc ) .
carbon finance has been held up for its potential to yield 
 cobenefits 
 by enabling access to improved cookstove technologies by poor households .
while flagging carbon markets and evidence from early efforts call 
 the near - term practicality of this into question , it remains an important 
 possible source of finance .
current carbon finance methodologies include 
 greenhouse gases ( ghgs ) but fail to account for the climate impact 
 of pics such as bc , co , oc , and non - methane hydrocarbons ( nmhc ) , mainly 
 because of the high uncertainty and variability in their emissions and the differing spatial and temporal scales of their impacts relative 
 to ghgs .
bc dominates cookstove pic climate impacts and has become a focus for mitigating near - term climate change . 
 in response
, the gold standard foundation recently developed a simplified 
 methodology , which relies on either laboratory 
 or ( preferred ) field - based emission measurements , with the latter 
 collected using the kitchen testing protocol ( kpt ) , to incentivize bc mitigation efforts .
however , the gold 
 standard does not require field emission measurements 
 to be made ( only measurements of fuel use reductions ) , and as noted above few cookstoves have actually 
 been measured in the field .
therefore , an important missing piece 
 is a rigorous understanding of in situ emissions from cookstove technologies 
 and the extent to which the emission reductions indicated by laboratory 
 testing are achieved under real - world conditions .
to address 
 this gap , we conducted an evaluation of in - field emissions 
 in malawi , africa focusing on two pre - existing cookstove programs . 

the specific objectives of this work were : 1 ) to measure fuel use 
 and emission factors from in - home use of alternative and baseline 
 household and institutional cooking technologies ; 2 ) compare emission 
 factors and rates with existing measurements and emission  tiers  ; 
 3 ) analyze real - time optical properties ( absorption and scattering ) 
 of aerosols during in - home use ; and 4 ) estimate and compare health 
 and climate impacts / benefits suggested by lab and field - based measurements .
malawi 
 is a small , landlocked country in southeastern africa in which over 
 90% of the population uses biomass as their main source of domestic 
 energy .
it is one of the most densely populated 
 countries in sub - saharan africa and among the poorest in the world , 
 ranking 173 among 188 countries in human development index .
high poverty rates , dependence on unsustainably 
 harvested firewood , and a predominantly rural population means there 
 is a great need for improvements in household energy systems and makes 
 malawi an ideal location to study the impacts of alternative cookstove 
 technologies .
household emission measurements of uncontrolled 
 in - home cookstove use ( following the kpt protocol ) took place during routine cooking activities in september - october 
 2015 ( the hot and dry season ) .
emission tests were completed in two 
 communities on cookstoves at opposite ends of the technology spectrum 
 discussed above . in one ,
the cooking and pneumonia study ( caps ; www.capstudy.org ) , led by the 
 liverpool school of tropical medicine ( lstm ) , distributed limited 
 quantities of two fdcs models ( philips hd4012ls and ace-1 ; cost 90 
 usd ) as part of pilot activities for this community - level randomized 
 controlled trial of the effects of the philips hd4012ls cookstove 
 on the incidence of pneumonia in children under the age of 5 .
fdcs use was not widespread in this community ; 
 on the order of 1020 cookstoves had been distributed . in the 
 other ,
the nongovernmental organization ( ngo ) concern universal ( cu ; www.concern-universal.org ) helped establish nearly universal distribution of the chitetezo 
 mbaula ( cm ) cookstove , a low - cost ( 12 usd ) , locally 
 produced , natural - draft clay cookstove , with the main objective of 
 reducing fuel use by users and with funding from the sale of carbon 
 credits . in both communities , traditional 
 three stone fires or simple mud stoves
cookstoves are 
 shown in figure s1 in the supporting information ( si ) . in - home cookstove use
included cooking of traditional foods 
 such as nsima ( a corn - flour porridge , the staple 
 food of malawi ) or rice , preparing vegetables or meat , and heating 
 water for bathing and took place inside , in semicovered verandas and 
 outside .
in addition to in - home testing , controlled cooking 
 tests ( cct ) were conducted on several 
 larger , wood - burning 
 institutional cookstoves being piloted at an orphanage .
institutional 
 cookstoves are used where a large number of people are fed ; they are 
 much larger than household cookstoves to accommodate a dedicated , 
 large cooking pot ( 80100 l ) that sits inside the stove body , 
 enabling more efficient heat transfer .
institutional cookstoves tested 
 included a large institutional three stone fire ( i - tsf ) , aleva ( al ) , 
 mayankho ( ma ) , and the jumbozama ( jz ) .
the jumbozama is a scaled - up 
 version of the zama zama rocket gasifier cookstove ( rocket works , 
 durban , south africa ) built inside a masonry housing .
figure s2 ( si ) shows the institutional cookstoves tested , 
 and table s1 summarizes tests conducted during the campaign . in -
home emission 
 measurements were performed using the portable stove emissions measurement 
 system ( stems ; figure s3 in the si ) , which 
 utilizes the  sensor board  from a portable emission 
 measurement system ( aprovecho research , cottage grove , or ) .
the stems 
 runs on a 12 v battery and measures real - time ( 2 s ) concentrations 
 of carbon dioxide ( co2 ) , carbon monoxide ( co ) , temperature , 
 relative humidity ( rh ) , and particle light scattering ( bsp ; also used as a proxy for real - time pm2.5 mass concentration ) 
 with a laser photometer ( optical wavelength ,  = 635 nm ) .
integrated filter samples were 
 collected on two 47 mm diameter filter trains with equal flows for 
 gravimetric and thermo - optical oc / ec analysis ( see the si for details ) .
one of the filter trains contained 
 a quartz filter , and the other contained a teflon filter followed 
 by a backup quartz filter downstream to correct for gas phase absorption 
 artifacts .
additional details on the 
 stems sensors , filter analysis and associated uncertainties , and quality 
 assurance are provided in section s1 in the si .
real - time pm light absorption at  = 880 nm was measured 
 using an ae-51 microaeth ( aethlabs ) incorporated within stems . to 
 avoid excessive filter loadings and frequent filter ticket changes 
 in the field , an external flow meter ( honeywell awm3150v ) and vacuum 
 source were used in place of the internal pump and flow rate set at 
 1025 cm min .
the microaeth 
 filter loading artifact was corrected via the algorithm described 
 by park et al .
a six - armed 
 stainless - steel probe with sampling ports radially 
 centered in equal areas was used to capture a representative sample 
 of naturally diluted emissions approximately 11.5 m above 
 the cookstove . from the probe
, emissions 
 passed through conductive sampling tubing to the stems via a 2.5 m 
 cut - point cyclone ( bgi inc . ) .
wood fuel was set aside 
 before the start of cooking and wood moisture and weight recorded 
 as per the kpt protocol .
wood moisture 
 content was measured with an electronic moisture meter ( lignomat mini - ligno 
 s / dc ) .
the fire was started using matches or hot charcoal 
 left from a previous cooking session ; the latter practice was more 
 common .
one kg of wood can result in up to 161 g of char being 
 formed .
neglecting starting char likely 
 biases high the estimates of wood consumed , whereas neglecting left - over 
 char results in a low bias to wood consumed . to account for this , 
 we assume a conservative 20% uncertainty in wood consumed .
a brief , 
 anonymous survey was conducted after testing to collect user feedback 
 on performance and perception of alternative cookstoves .
fuel 
 based emissions factors were calculated using the carbon balance 
 method , assuming that carbon comprises 50% of dry wood by weight and 
 all gaseous carbon in the wood is emitted as co and co2 . since the summed carbon mass obtained from background - corrected 
 co and co2 concentrations serves as a tracer for the fuel 
 consumed , the carbon balance does not require all emissions to be 
 captured .
other carbonaceous species ( e.g. , gaseous hydrocarbons ) 
 contribute a relatively small fraction ( < 5% ) of carbon in emissions 
 and are neglected in this calculation .
additional details 
 on ef and er calculations and associated uncertainties are described 
 in si section s3 .
45 household cooking sessions were measured , 
 with durations from 
 19 to 233 min ( median = 49 min ) across 4 cookstove technologies ( traditional , 
 philips , ace-1 and cm ) in 22 households , with two tests ( in some cases 
 with different cookstoves ) conducted in each household wherever possible . 

axis labels in figure 1 indicate the number of tests of individual cookstove types ; cookstove - type , 
 household identifier , and emissions data from all tests are listed 
 in table s5 .
wood moisture ranged from 
 6 to 26% for all sessions and was not significantly different for 
 tests of different cookstoves .
traditional cookstoves had the highest pm ef of 7.8 
  2.9 g kg ( average  sd ) , similar to 
 field observations in honduras , while 
 philips had the lowest ( 4.1  0.6 g kg ) , 
 with 47% lower mean emissions than the traditional stove , a statistically 
 significant reduction ( p < 0.005 from two - sample t test ) .
mean cm and ace-1 efs were lower than that from 
 traditional stoves but not significantly so ( p = 
 0.347 and 0.158 , respectively ) .
linear regressions showed no relationship 
 between fuel moisture content and pm ef for individual cookstove types 
 ( r < 0.1 for all ) .
box plots of pollutant 
 emission factors ( in g kg ) and emission rates 
 ( in mg min ) , with the number 
 of tests of each type indicated in the axis label .
boxes represent 
 interquartile range , whiskers indicate 5th and 95th percentiles , and 
 the horizontal line in the box is the median .
95th percentile whiskers 
 for some of the cookstoves were out of scale on the y axis and are indicated by numbers on the top axis .
panel a : pm2.5 ef ; panel b : co ef ; panel c : pm er , panel d : co er ; panel 
 e : ec ef ; panel f : ec er ; panel g : ec / tc ratio ; panel h : single scattering 
 albedo at 880 nm .
tier values for panels c and d were taken from iso 
 iwa 11:2012 guidelines .
the legend for 
 all panels is in panel a. an asterisk before the stove name on the 
 lower axis indicates a statistically significant difference with respect 
 to traditional stoves ( p < 0.05 ) .
data from shen 
 et al . is for a movable metal cookstove 
 used in china .
co efs ( figure 1b ) measured ranged from 28 to 198 g kg , with 
 traditional stoves the highest ( 98  26 g kg ) , comparable to field measurements from traditional open fires in 
 honduras ( 116  55 g kg ) .
both fdcs models had statistically significant reductions 
 of 45% ( p < 0.005 ) .
the mean co ef for cm was 
 106 g kg , not significantly different from traditional 
 stoves ( p = 0.507 ) .
although these efs are 
 similar to those from other field studies , 
 they are considerably higher than those observed in laboratory tests . 
 mean pm2.5 ef for traditional stoves is roughly 3.7 times 
 higher than laboratory tests .
for the 
 philips , laboratory mean pm2.5 and co efs were 80 and 65% 
 lower than our mean values , respectively .
estimated uncertainties in our pm ef values are 1030% , with 
 an average uncertainty of 15% ( si section 
 s3 ) , smaller than variability in ef values within groups ( e.g. , pm 
 ef coefficient of variation ranges from 14% for philips to 44% for 
 cm ) .
fuel based efs do not account for two parameters important 
 for 
 understanding total emissions : cooking time and wood consumed , which 
 depend on cooking activity and cookstove efficiency . here
figure 
 s5 in the supporting information shows 
 a box - whisker plot of wet - basis ( as measured ) wood consumption rate 
 in kg h. traditional stoves had the highest fuel 
 consumption rate ; mean reductions from the cm , ace , and philips were 
 26% , 27% , and 51% , respectively .
observed reductions are consistent 
 with , but slightly smaller than , reductions in wbt fuel consumption 
 observed during tests of the cm ( 33% ) and philips ( 61% ) in malawi .
figure 1c and 1d show the pm and co 
 ers estimated using measured 
 fuel values ( on a dry fuel basis ) ; also shown are tier boundaries 
 for indoor emission rates .
the philips shows reductions 
 of both pm2.5 and co ers by 70% compared to traditional 
 stoves
. however , pm and co ers based on laboratory testing of philips 
 cookstove ( for wet wood ) are 76 and 61% , respectively , lower than 
 our mean values .
this indicates that 
 laboratory tests may substantially underestimate real - world emissions 
 of even the most advanced wood - burning cookstoves when fuel properties 
 and cookstove operation are variable .
figures 1e to 1 g show box plots 
 of ec efs , ers , and ec / tc , respectively , 
 where tc is total carbon ( oc+ec ) .
all quantities show high variability 
 due to the uncontrolled nature of this combustion and varying usage 
 and fueling of the cookstoves and skill of the cook .
ec efs and ec / tc 
 ratios for intervention cookstoves are generally higher than those 
 for traditional stoves , though only the philips shows a significant 
 difference for ec / tc .
increases are moderated for ers due to reduced 
 fuel use , especially for the philips cookstove .
ec efs for cm were 
 similar to observations for basic  improved  cookstoves 
 in other field studies .
fdcss had the highest 
 ec / tc ratios ( 0.48 ) followed by cm ( 0.42 ) and tsf ( 0.28 ) .
ec / tc is 
 typically 0.1 for open biomass 
 burning , significantly lower than values observed in this study .
this 
 highlights that combustion under these relatively controlled conditions 
 emits particles with distinct properties compared to those from open 
 biomass burning .
additional discussion on ec / tc ratios and ssa is 
 provided in section 3.3 .
a summary of efs and optical properties for each test is provided 
 in si table s5 .
food- and fuel - based efs 
 for institutional cookstoves are reported in table 1 .
efs for pm and co followed a consistent 
 trend , with the i - tsf the highest , followed by the al , ma , and jz . 
 co and pm emissions were reduced by similar amounts , with co ( pm ) 
 emissions reduced by 67% ( 68% ) and 73% ( 75% ) for ma and jz , respectively . 

averaged wood consumption rate was highest for the aleva ( 5.2 kg h ) , followed by i - tsf ( 4.18 kg h ) , mayankho ( 3.85 kg h ) , and jumbozama ( 3.30 
 kg h ) .
food based pm and co efs follow the 
 same trend as their fuel based counterparts , with jz exhibiting > 80% 
 reductions for both .
the jz cookstove used a slightly smaller , different 
 cooking pot ( 80 l vs 100 l for other cookstoves ) , which may have contributed 
 to mean pm ef values ( e.g. , due to a quicker warm - up phase ) and cooking 
 times that were 61% and 32% lower , respectively , than those for other 
 cookstoves .
further extensive comparisons of these data are complicated 
 by the small sample size and lack of emission data in the literature .
figure 
 s6 in the si shows real - time ( 2 s and 1 
 min average ) concentrations of co , co2 , particle absorption , 
 and scattering coefficients ( bap ; bsp ) for representative 
 fdcs and traditional household stove tests .
gravimetric pm2.5 concentrations correlated well ( r = 
 0.87 ) with averaged bsp , suggesting that real - time scattering 
 was a reasonable proxy for real - time pm2.5 mass concentrations 
 under these test conditions ( si figure 
 s7 ) .
figure s6 also shows single 
 scattering albedo ( ssa ; fraction of scattering to total extinction ; 
 ( bsp/(bap+bsp ) , here at  = 
 880 nm ) and modified combustion efficiency ( mce ; co2/(co+co2 ) , where  indicates background - corrected 
 concentrations in ppm ) .
a lower ssa signifies a greater contribution 
 from absorption to total aerosol light extinction , while higher mces 
 indicate more efficient combustion .
all cooking events were characterized 
 by a scattering spike at startup ( evident 
 in figure s6 ) .
observations of cooking 
 activity showed that addition and adjustment of fuel typically resulted 
 in spikes in bap for fdcs and in bsp for traditional 
 stove tests .
fdcs tests typically had a large scattering peak only 
 at startup and overall particle extinction was dominated by absorption , 
 while extinction from traditional stove tests was dominated by scattering . 

as a result , traditional stove tests had comparatively higher ssa 
 ( shown in figure 1h ) 
 and lower mce than fdcs tests .
test - average ssa was highest for traditional 
 stoves ( 0.36 ) , followed by cm ( 0.28 ) and fdcs ( 0.25 ) ; particles from 
 all  alternative  cookstoves are more absorbing than 
 those from traditional stoves and thus have greater specific warming .
our optical measurements are at  = 880 nm , which influences 
 the quantities ( e.g. , ssa ) presented here , but likely not the relative 
 trends discussed here and below . for example , while ssa for pure bc 
 aerosol is typically 0.150.3 at  = 530 nm , we observe periods with ssa ranging from 00.2 
 for fdcs , despite the fact that the particles are not pure bc .
section 
 s5 and figure s8 in the si present mie 
 theory modeling indicating that both the longer wavelength and smaller 
 particle diameter in fdcs emissions likely strongly reduces the scattering 
 efficiency of these particles . the average mass scattering cross section 
 ( msc ; ratio of scattering to gravimetric pm2.5 concentration ) 
 over all tests was 0.87  0.31 m g , substantially lower than mscs of 3.64.3 m g (  = 550 nm ) and 2.2  0.6 m g (  = 530 nm ) reported for emissions 
 from dry biomass burning and in - field cookstoves , respectively .
lower msc values in our study are consistent 
 with the wavelength dependence of scattering . averaged mass absorption 
 cross - section ( mac ; ratio of bap to ec concentration ) for 
 all tests was 13.2  4.8 m g , 
 overlapping with the mac of 12.5 m g assumed in the microaethalometer .
the data show a general trend of decreasing ssa with increased 
 mce and ec / tc .
si figure s9 shows the relationships 
 between mce and ec / tc with ssa for in - home cookstove testing .
greater 
 specific absorption correlates somewhat with higher mce ( r = 0.29 ) and more strongly with higher ec / tc ( r = 0.57 ) .
these relations are consistent with greater bc 
 production in more efficient , contained combustion where mce is highest , 
 while more scattering oc is produced during less efficient ( perhaps 
 lower temperature ) combustion . relationships between mce , ec / tc , and 
 ssa have been proposed based on measurements of open biomass burning 
 for a range of fuels .
while the general trends we observe 
 are consistent with the published parametrizations , a direct comparison 
 is not possible due to differences in measurement wavelength .
however , 
 it is likely that the relationships would be different because combustion 
 technology strongly influences aerosol properties .
test - average 
 quantities do not reflect the contribution of distinct 
 combustion phases to total emissions .
analysis of real - time data can 
 give insight into the variation of parameters such as mce and particle 
 properties throughout a burn .
patterns 
 of real - time emissions data ( parted ) analysis approach of 
 chen et al . to evaluate quantities and 
 optical characteristics of emissions based on real - time data . in this 
 analysis , mce and ssa are calculated for each minute of data ( termed 
 a combustion event ) .
a bivariate histogram of mce and ssa , weighted 
 by instantaneous scattering emission factor ( iefscat , particle 
 light scattering normalized by mass of fuel consumed ) and normalized 
 by total scattering emissions , is then constructed .
the resulting 
 plot shows the fractional contribution of combustion at specific conditions 
 ( mce ; ssa ) toward total scattering emissions .
this weighting is chosen 
 to represent the distribution of total particle emissions , as scattering 
 shows a strong correlation ( r = 0.87 ) 
 with pm2.5 from gravimetric analysis ( figure s5 ) .
figure 2 shows parted 
 plots for traditional , cm , and fdcs ( ace-1 and philips combined ) emission 
 tests , with each panel representing all test data from that cookstove 
 type .
all display a cluster at mce > 0.9 and ssa < 0.4 , 
 suggesting 
 that all produce more absorbing particles during more efficient flaming 
 combustion .
however , all test types also had combustion events and 
 particle emissions at lower mce and higher ssa , with noticeably more 
 spread in this direction among the traditional cookstove and cm tests . 

the fdcs plot shows a more distinct and concentrated cluster at high 
 mce , low ssa .
figure s10 in the si shows 
 histograms weighted with fuel consumption ( calculated as co+co2 ) rather than particle scattering and shows that the vast 
 majority of fuel consumption occurs at high mce / low 
 ssa ( 88% at mce > 0.9 and ssa < 0.4 for fdcs ) , whereas only 
 12% 
 of scattering emissions took place under these conditions .
the corresponding 
 fractions ( fuel consumption / scattering ) in this mce - ssa range are 
 57%/15% and 58%/25% for the cm and traditional tests , respectively . 

this reinforces that the relative distribution of combustion conditions 
 varies between stoves and has substantial impacts on particle emissions 
 and properties .
( though less so for traditional / cm tests ) 
 but emit a large fraction of scattering particles / particle mass .
time 
 series data ( figure s6 ) show that all tests 
 had sharp peaks in scattering emissions during cookstove startup , 
 consistent with other field measurements .
the parted plots also suggest that these startup emissions may make 
 outsized contributions to overall pm emissions .
we examine the contribution 
 of startup emissions in si figure s11 , 
 which plots the running average of iefscat against normalized 
 time for each test of a cookstove type . in all tests , the average 
 peaks strongly during the startup phase , confirming the importance 
 of start emissions . although the test averaged iefscat ( at 
 the right edge of the graph )
is highest for traditional and lowest 
 for fdcs tests , we see a much higher peak for gasifiers and in nearly 
 all cases a monotonic decrease in iefscat during the test , 
 reinforcing the dominance of startup emissions for these cookstoves . 

the outsized contribution of startup to pm emissions from fdcs has 
 important implications for exposure as during startup the cook is 
 assured to be in close proximity to the cookstove .
bivariate histogram of 
 mce and ssa weighted by particle emissions 
 ( parted plots ) for a. traditional ; b. chititezo mbaula , and c. fdcs . 

the bottom axis delineates bins of single scattering albedo ( ssa ) 
 at 880 nm , and the left axis shows modified combustion efficiency 
 ( mce ) bins .
each location on the plot represents an ssa and 
 mce at which a combustion event ( 1 min ) may occur ; the color scale 
 indicates the percent contribution of emissions at that condition 
 to the total scattering ( pm mass ) emitted .
three of the traditional 
 stove tests were excluded from this analysis due to lack of either 
 co2 ( 1 test ) or absorption data ( 2 tests ) .
100- and 20-year gwc ( tons 
 of co2-equivalent per year of cookstove use ) were estimated 
 based on measured pollutant emission factors ( co2 , co , 
 oc , and ec ) for household and institutional cookstoves following the 
 approach of previous work , as briefly 
 discussed in si section s6 .
gwcs associated 
 with the use of a modern fuel , lpg , are also included as a benchmark 
 ( based on laboratory efs ) .
the intergovernmental panel on climate 
 change ( ipcc ) default value of 0.81 for the fraction of nonrenewable 
 biomass ( fnrb ) in malawi was assumed 
 for all calculations , though this is highly uncertain due to factors 
 including spatial heterogeneity in fnrb and uncertainties in data 
 on fuel demand and its dynamics .
a fixed 
 energy demand was assumed and annual fuel use for each cookstove estimated 
 based on fuel use rate reductions observed in this study ( figure s5 ) relative to the baseline ( traditional 
 cookstoves ) .
gwc calculations used global warming potential ( gwp ) 
 values recommended by the gold standard foundation and ipcc and account only for emissions during fuel combustion . 

gwc associated with upstream processes ( e.g. , fuel production and 
 transport ) related to cookstove fuel use have been found to be negligible 
 for woodfuel and relatively small ( 1020% of combustion stage ) 
 for lpg , though they may be considerable 
 for other fuels such as coal or charcoal .
ch4 makes a substantial contribution to the gwc associated 
 with cookstove use but was not measured here . to 
 approximate ch4 gwc
, ch4:co ratios from the 
 literature were used to estimate ch4 efs ; ch4:co ratios of 0.05 for ace-1 and philips 
 and 0.08 for the traditional , cm , and institutional cookstoves were 
 used in calculations .
gwc contributions from other hydrocarbons and 
 n2o are small for biomass emissions and are neglected here . also not included 
 in this accounting is brown carbon ( brc ) , the component of oc that 
 absorbs energy across visible and ultraviolet wavelengths .
we did 
 not measure absorption at multiple wavelengths in this work but expect 
 that the brc absorption would make a relatively small contribution 
 to short - wavelength absorption considering the very high ec : oc ratios 
 ( mean for all tests was 1 ) observed in emissions from all 
 cookstoves .
brc is expected to have little additional impact on climate 
 forcing for emissions with ec : oc ratios above 0.1 , though this merits further study .
figure 3 shows the gwc values estimated 
 across a 100-year horizon ( figure s12 shows 
 20 year gwcs ) .
as expected , traditional cookstoves show the highest 
 gwc , followed by the cm , ace , and philips . across a 100 year horizon 
 for household cookstoves , cm , ace , and philips show overall reductions 
 of 13% , 23% , and 55% , respectively , from the household traditional 
 ( hh - trad ) stoves . across household stove models ,
the highest relative 
 contribution by species is from co2 ( 5169% ) followed 
 by ec ( 2638% ) with other species contributing less than 10% . 

calculations using laboratory efs suggest much larger reductions ( 59% 
 and 93% for improved and fdcs , respectively ) relative to traditional 
 stoves , highlighting again the implications 
 of the lab - field discrepancy in emissions . due to high combustion 
 efficiency for lpg
, pics contribute minimally to its gwc , and the 
 vast majority is contributed by co2 .
the gwc of lpg is 
 a factor of 4 lower than the cleanest cookstove tested in the field 
 ( philips ) . among institutional cookstoves ,
only the jz yielded substantial 
 fuel use and emission reductions during our limited testing , leading 
 to an 50% reduction in gwc relative to baseline . across a 
 20-year horizon ( figure s12 ) ec contributes 
 the most to biomass cookstove gwc , contributing 5365% across 
 all cookstoves .
100 year gwc values for one year of use of in - home cookstoves 
 ( l ) 
 and institutional cookstoves ( r ) from major short- and long - lived 
 climate forcing species emitted by cookstoves .
the ch4 component 
 is estimated based on the ch4:co ratio from other studies . 

note that the daily energy use is different for household and institutional 
 cookstoves . in - home cookstoves
institutional cookstoves are assumed to be used for twice 
 a day for 5 days a week for 40 weeks a year with an energy basis based 
 on average fuel use measured in this study .
figure 4 combines 
 gwc estimates with those for human exposure to pm2.5 to 
 examine the cookstoves measured in a 
the exposure estimation applies an individual intake fraction of 1300 
 ppm ( 1 ppm = 1 mg inhaled per kg emitted ) to link emissions to human 
 exposure .
the figure shows estimated 
 daily pm intake ( horizontal axis ) and gwc ( vertical axis ) of several 
 cookstove technologies evaluated in that study , with added estimates made based on data from in - home testing 
 during this study .
it should be noted these calculations assume complete 
 adoption of the cookstove in question , while field trials have shown 
 that new technologies are rarely used exclusively and 
the exposure - response relationship for all - age mortality risk from 
 ischemic heart disease ( ihd ) from burnett et al . is used to estimate adjusted relative risk of mortality 
 due to ihd ( dose  response for chronic obstructive pulmonary 
 disease mortality is similar ) , shown on the lower horizontal axis . 

this extrapolation of our field emissions data suggests that the philips 
 cookstove reduces pm emissions and intake by approximately 75% relative 
 to the traditional stove ; this is associated with a smaller reduction 
 in estimated mortality relative risk ( from 2.2 to 1.9 ) due to the 
 nonlinear dose 
this figure dramatically demonstrates the implications of the performance 
 decrement observed in field measurements .
field - tested biomass stoves 
 do not meet expectations based on laboratory tests in terms of emissions / exposure . 
 for example ,
relative to estimates based on laboratory measurements , 
 the field - measurement - based daily pm intake and gwc for the philips 
 are a factor of 8.6 and 2.8 times higher , respectively .
the philips 
 and jz have the lowest estimated intake and gwc among in - home and 
 institutional cookstoves , respectively , but are still associated with 
 far greater impacts than the  benchmark  lpg cookstoves . 

for example , compared to the estimated impacts of lpg cookstove use , 
 in - home use of a philips cookstove results in 4.9 times higher gwc 
 and around 66 times higher daily exposure , corresponding to increase 
 in adjusted relative risk for ihd mortality from 1.25 to 1.95 .
impact 
 estimates for hh - trad , cm , and three of the institutional cookstoves 
 ( i - tsf , ma , and al ) are within the bounds of impact estimates for 
 laboratory - tested traditional stoves , whereas the best performing 
 fdcs is in the range estimated based on laboratory performance of 
 a basic improved biomass cookstove ( w - im - u ) . field observations of 
 emissions from a range of cookstoves used in interventions give important 
 insights into the potential for these technologies to mitigate the 
 health and climate impacts associated with traditional cookstoves .
health 
 and climate impacts of various cookstove - fuel combinations 
 based on laboratory emission test data ( shown with circles ; adapted 
 from grieshop et al . with fuel renewability 
 and energy demand values described in text ) along with the estimates 
 from this study are shown , marked with diamonds and squares with error 
 bars for in - home and institutional cookstoves , respectively .
abbreviations 
 for laboratory - based calculations : w - tr - u : wood - burning traditional 
 unvented cookstoves ; w - im - u : wood - burning improved unvented cookstoves ; 
 w - gas - u : wood - burning gasifier unvented cookstoves ; w - fan - u : wood - burning 
 fan unvented cookstove .
household traditional ; cm : chitetezo mbaula ; philips : philips 
 hd4012ls ; ace : ace-1 ; i - tsf : institutional three stone fire ; al : aleva ; 
 ma : mayankho ; jz : jumbozama .
our results suggest that 
 both simple  improved  cookstoves 
 and more advanced biomass cookstoves provide some benefits but fall 
 short of those indicated by laboratory testing or that may be possible 
 through the use of modern fuels / devices .
impact estimates shown here 
 ( figure 4 ) are rough 
 approximations but are consistent with field trials that have seen 
 less than expected benefits from cookstove interventions .
for example , the fact that no effect on childhood pneumonia incidence 
 was observed during caps may be partly 
 due to poorer - than - expected performance of these cookstoves under 
 real - world conditions ; other factors such as continued use of traditional 
 stoves and exposure to air pollution from other sources likely also 
 contributed to this outcome .
forthcoming findings from other intervention 
 trials will report on the health effects associated with other fuel / cookstove 
 technologies including lpg and give insights into whether or not they 
 provide the benefits that might be predicted .
a range of factors contribute to reduced performance observed 
 in 
 the field , all related to the difficulty of controlling combustion 
 of heterogeneous fuels under widely ranging conditions .
part of the 
 performance decrement observed is due to cookstoves not being used 
 in accordance with manufacturer recommendations .
for example , wood 
 pieces sticking out of the top of fdcs were commonly observed in the 
 field ( figure s1 in the si ) and lead to 
 suboptimal combustion of the volatiles emitted from pyrolysis of wood . 
 however , this practice is unsurprising when one considers that processing 
 larger logs and branches to the size recommended for the fdcs models 
 considered here ( 1  5 cm ) represents additional work 
 for the household .
this source of variability may be addressed via 
 a cookstove that is highly robust to changing fuel type / configuration 
 or a situation in which a homogenized fuel source ( e.g. , pellets ) 
 is provided or readily available .
the former is only likely possible 
 in a more advanced combustion device ( e.g. , the enclosed heating stoves , 
 often with catalytic after treatment of exhaust used in developed 
 countries ) that are beyond the budget of the target population , while 
 the latter requires a close look at the broader system , beyond the 
 cookstove .
these observations highlight the need to expand the 
 view beyond 
  clean cookstoves  to clean and controlled cooking systems , 
 which could provide considerable health and climate benefits and perform 
 consistently and reproducibly under both laboratory and field conditions . 

one approach advocated is to focus efforts on a switch to modern appliances 
 ( e.g. , electrical induction cookers and lpg ) rather than promoting 
  improved  biomass cookstoves , though such technologies would be out of reach in the short- to 
 medium - term for the poorest of the world s poor ( e.g. , rural 
 malawians ) .
improvements using biomass can likely be made by improving 
 the cookstove / fuel system in tandem .
our findings emphasize 
 that laboratory protocols do not fully anticipate 
 real - world emissions when fuel properties and cookstove operation 
 are variable and highlight the need for testing approaches that more 
 accurately represent real - world cookstove use .
field evaluation of 
 emissions performance early in product development would be one way 
 to achieve this ; another way would be to develop testing protocols 
 that 
 simulate the range of ways a cookstove may be used , including low - efficiency 
 emissions such as smoldering , often observed in field emissions .
these data were collected during 
 a dry season , and fuel moisture may also have important impacts on 
 stove performance ; future field studies 
 should assess stove performance across an annually representative 
 period if possible .
 emissions 
 from traditional cooking practices in low- and middle - income 
 countries have detrimental health and climate effects ; cleaner - burning 
 cookstoves may provide  co - benefits  . here 
 we assess 
 this potential via in - home measurements of fuel - use and emissions 
 and real - time optical properties of pollutants from traditional and 
 alternative cookstoves in rural malawi . 
 alternative cookstove models 
 were distributed by existing initiatives and include a low - cost ceramic 
 model , two forced - draft cookstoves ( fdcs ; philips hd4012ls and ace-1 ) , 
 and three institutional cookstoves . among household cookstoves , emission 
 factors ( ef ; g ( kg wood)1 ) were lowest for the 
 philips , with statistically significant reductions relative to baseline 
 of 45% and 47% for fine particulate matter ( pm2.5 ) and 
 carbon monoxide ( co ) , respectively . 
 the philips was the only cookstove 
 tested that showed significant reductions in elemental carbon ( ec ) 
 emission rate . 
 estimated health and climate cobenefits of alternative 
 cookstoves were smaller than predicted from laboratory tests due to 
 the effects of real - world conditions including fuel variability and 
 nonideal operation . 
 for example , estimated daily pm intake and field - measurement - based 
 global warming commitment ( gwc ) for the philips fdcs were a factor 
 of 8.6 and 2.8 times higher , respectively , than those based on lab 
 measurements . 
 in - field measurements provide an assessment of alternative 
 cookstoves under real - world conditions and as such likely provide 
 more realistic estimates of their potential health and climate benefits 
 than laboratory tests . 
Introduction
Methods
Results and Discussion
Implications