transportation in nyc 
 mainly comprises private vehicles , taxis , 
 and a large network of public transportation that includes aboveground 
 trains , the subway system , and buses . of these , the nyc subway is 
 a preferred mode of transport for many commuters . on an average weekday , 
 over 5 million
therefore , the air quality conditions in the 
 subway stations may have a significant impact on a large population 
 of commuters as well as subway workers . over the years
, many 
 studies in cities that have subways have found 
 that air pollutant concentrations in subways are several times higher 
 than aboveground levels .
pm2.5 concentrations 
 in the london and stockholm subway systems were approximately 10 times 
 higher than aboveground .
health effects of ambient 
 fine particulate matter ( pm2.5 ) have been studied in detail , 
 and epidemiological studies have established that pm2.5 pollution is a major risk to human health .
one 
 of the earliest of such studies , the harvard six cities study , showed 
 that long - term exposure to particulate matter air pollution resulted 
 in an adjusted mortality rate ratio of 1.26 per range of 11.0 to 29.6 
 g / m ( 95% confidence interval , 1.081.47 ) 
 with fine particulate matter .
short - term 
 exposure to increasing levels of pm2.5 is also associated 
 with acute effects that lead to respiratory and cardiovascular problems , 
 thereby causing increased hospitalization as seen in recent studies . since 2004 ,
nyc subway studies have shown levels of pm2.5 , 
 iron , 
 manganese and chromium to be several times higher than aboveground 
 levels .
these studies showed that steel dust is the primary 
 source of iron , manganese , and chromium exposure in teenage students 
 and workers who participated in the studies .
these pm levels and composition 
 in the nyc subway system can be partially explained by the fact that , 
 during operation , friction results in the release of particles from 
 the metal - to - metal contact between the car wheels and the rail .
it is important to note , however , that other factors can affect 
 air quality in the subway system , such as the exchange of air between 
 the road level and subway , as well as the use of a variety of operational 
 equipment in the subway system . upon preliminary investigation
, we 
 determined that the nyc subway system uses diesel - powered locomotives / cars 
 for maintenance work at night .
the use of diesel engines can inevitably 
 lead to the release of diesel soot , including bc , but the levels of 
 bc in the subways have not been previously examined .
the main 
 goal of this study was to monitor the exposure levels 
 of bc and pm2.5 in the subway system in nyc , with a focus 
 on manhattan , the most populous area , and assess if these levels are 
 in the range that may cause adverse health concern .
specific objectives 
 of the study included : ( 1 ) measuring levels of bc in selected subway 
 stations along subway lines in manhattan ; ( 2 ) simultaneously measuring 
 levels of pm2.5 in the same subway stations and comparing 
 them with bc levels ; ( 3 ) comparing bc and pm2.5 exposure 
 levels in the underground subways with above ground road exposure 
 levels ; and ( 4 ) studying the spatial and temporal variations in bc 
 and pm2.5 in the subway system .
this research was an exposure assessment study in which concentrations 
 of bc ( bc2.5 m size fraction ) and pm2.5 were simultaneously measured using personal air samplers , during 
 the period of january 2013 to march 2014 , in selected subway stations 
 along 5 subway lines in nyc .
this personal monitoring was performed 
 to sample airborne concentrations that workers and commuters may experience 
 during work or daily commutes in the nyc subway system .
bc and pm2.5 concentrations 
 were measured using both real time personal air sampling instruments 
 and standard filter - based methods .
the equipment were carried in a 
 backpack , and switched on before each sampling run . bc concentrations 
 were measured in real time using a micro - aetholameter ( magee scientific / aeth 
 laboratories , san francisco , ca ) and pm2.5 concentrations 
 were measured using a nephlometric - based real time dataram ( dataram 
 pdr 1500 , thermo scientific , franklin , ma ) .
both real time machines 
 were set to log data at 1 min intervals giving continuous 1 min average 
 readings as the output .
a 2.5 m cut point cyclone was used 
 with the micro - aethalometer and pdr 1500 dataram for the measurement 
 of 2.5 m diameter and below size component of bc and pm respectively . 

all real time reading instruments had been factory calibrated prior 
 to use in the study .
the pdr 1500 datarams were autozeroed with filtered 
 air and compared for interinstrument variability prior to each sampling 
 run filter - based methods were also used to sample for particulate 
 matter 
 at some stations for comparison with the real time data .
a 2.5 m 
 cut personal environmental monitor ( pem ) ( skc , shoreview , mn ) was 
 used to collect particulate matter samples on prebaked 37 mm quartz 
 filters at 4 or 10 l / min . the mass concentration of elemental carbon 
 ( ec ) was measured from these filters using the lab ocec aerosol analyzer 
 following the niosh ( 2003 ) method 5040 ( sunset instruments inc . ,
pm2.5 samples were also collected on 37 mm teflon 
 ( pall , ann arbor , mi ) filters with a 2.5 m cut personal environmental 
 monitor . from the teflon filters , pm2.5 mass concentrations 

were measured using standard gravimetric analysis in the nyu sterling 
 forest laboratory temperature- and humidity - controlled weighing room .
a total of 35 subway platforms in 
 30 subway stations located along 4 subway lines in manhattan and 1 
 subway line in queens were selected for monitoring of bc and pm2.5 levels .
individual as well as group sampling were carried 
 out at selected subway stations and lines .
for individual sampling , 
 concentrations at selected single stations and stations along selected 
 lines were monitored by one person at different times .
a group sampling 
 run was also carried out in three different subway lines by three 
 individuals measuring concentrations simultaneously at each parallel 
 street crossing .
this was done to allow subway line comparisons that 
 minimized variations in concentrations due to time and other spatial 
 factors .
the west fourth street subway station located in downtown 
 manhattan was the primary station used for real time measurements 
 of bc and pm2.5 .
bc and pm2.5 concentrations were 
 measured starting while walking above ground for approximately 5 min , 
 followed by monitoring on the upper level and then the lower level 
 of the west fourth street station for 510 min each . the time 
 of entering the station , the time spent at the upper level , and the 
 time spent at the lower level were recorded . due to concerns 
 for temporal variation in measurements , in a group 
 study ,
bc and pm2.5 concentrations were systematically 
 recorded at stations along three different subway lines simultaneously 
 ( the parallel e , f , and 6 lines ) . after leaving from the same above 
 ground starting place ,
three individuals traveled in the uptown direction 
 of the subway and measured concentrations at six stations uptown from 
 the starting point . sampling was repeated at the same stations in 
 the downtown direction .
because the number 7 subway line has 
 stations situated at both 
 aboveground and underground levels in the same line , it was selected 
 for comparison of aboveground vs underground concentrations .
the 7 
 line runs aboveground in queens with underground stations located 
 more toward manhattan .
data were analyzed 
 using the minitab 
 version 16.2.4 statistical software package to test differences in 
 pm and bc concentrations at the individual stations and stations along 
 different subway lines . a student s t test 
 was used to analyze differences in levels while an analysis of variance 
 ( anova ) was used to analyze results from the group study .
mean bc2.5 ( with a 2.5 m cut size ) concentrations 
 ranged from 
 523 g / m ( each individual reading being 
 the mean of 510 min of sampling ) under normal conditions at 
 the stations sampled during the study period .
however , 1 min average 
 peaks as high as 111 g / m were recorded when a diesel 
 powered maintenance train passed .
the underground stations that had 
 the lowest three mean bc2.5 concentrations were 23rd st 
 ( e line ) , 34th st ( e line ) , and 8th st ( nqr line ) with means  

sds of 5.7  1.7 , 6.3  2.7 , and 7.5  5.4 g / m , respectively .
the highest three mean concentrations were 
 at 14th st ( f line ) , west 4th
st lower level ( f line ) , and astor place 
 ( 6 line ) with concentrations of 22.7  13.2 , 21.2  6.6 , 
 and 20.9  3.9 g / m , respectively .
bc 
 concentrations of all stations sampled during the study period are 
 given in table 1 .
the averaging time of each 
 reading was 510 min , depending on train arrivals .
a single measurement at a 
 station was the average of 510 min of sampling .
the numbers in parentheses next 
 to the concentrations is the number of times sampling was done .
only data from the group sampling runs and individual sampling of 
 the line were used to calculate the mean and sd of each station . note
: 
 concentrations given in [ box ] parentheses for 8th st and 34th st ( nqr 
 line ) are the mean  sd when a diesel maintenance train passed . 

the mean  sd bc2.5 concentrations of stations on the f , e , and 6 subway lines 
 ( average of means of all times sampled at the stations ) were 16.8 
  9.0 , 10.2  5.8 , and 15.5  4.3 g / m , respectively . the mean concentrations at stations on the f and 
 6 subway lines were not significantly different from each other .
mean 
 concentrations at each station along the three sampled lines are given 
 in table 2 .
each value 
 reported is the average 
 of two mean measurements where each measurement was a mean of 510 
 min of sampling .
the first measurement was taken in the uptown direction , 
 and the second was when the student reached the same station again 
 in the downtown direction .
the station s concentration 
 is significantly different from the concentrations of the other two 
 stations located at the same cross street .
the outdoor roadside ambient mean  sd bc2.5 concentration 
 measured just prior to the group sampling study was 2.3  1.2 
 g / m .
thus , the bc2.5 concentrations 
 at the stations along the f , e , and 6 underground subway lines were 
 several times higher than the roadside ambient bc2.5 concentrations 
 ( figure 1 ) .
bc2.5 concentrations at roadside 
 and in the stations 
 along the three parallel subway lines .
the box shows the interquartile 
 range , the line in the box shows the median , and the circle with number 
 shows the mean .
the whiskers represent the upper 25% and lower 25% 
 ( excluding outliers ) , while the * symbol represents outliers . the time series graph 
 ( figure 2 ) shows the 
 variations in bc2.5 concentrations at stations during an 
 individual sampling run along the nqr subway line .
graph showing change 
 in 1 min average concentration of bc2.5 when sampled starting 
 from outside the subway system and then at 
 six stations along the nqr subway line .
concentrations on the roadside ( outside the subway ) were 
 between 
 13 g / m , but these increased rapidly inside 
 the 8th st subway station . although concentrations at the next two 
 subway stations stayed below 20 g / m ( 1 min averages ) , 
 the levels rapidly reached a peak of 111 g / m at 
 the 34th st station .
concentrations at the next two stations 
 fell to levels observed prior to the passing of the diesel - powered 
 train . the mean bc2.5 concentrations 
 on the platforms of the upper and lower level of the west 4th st subway 
 station ( 6 paired samples ; each level 810 min ) were 14.0  
 5.4 and 20.9  3.8 g / m , respectively , and 
 were significantly different ( p < 0.05 ; figure 
 s1 , supporting information ) .
filter - based ec concentrations collected simultaneously 
 with real time aethalometer measurements for bc2.5 were 
 11.5 and 12.5 g / m at the astor place subway station 
 and 10.2 g / m at selected stations on the 6 subway 
 line . for comparison , the mean real time - based bc2.5 concentrations 
 simultaneously measured were 18.3 , 22.9 , and 12.5 g / m , respectively . the respective oc concentrations were 73.4 , 80.3 , 
 and 75.6 g / m ( table 3 ) .
similar 
 to the bc2.5 results , pm2.5 concentrations were 
 found to vary both spatially and temporally .
mean pm2.5 concentrations in underground subway stations ranged from 35200 
 g / m under routine conditions in the subway stations . 
 above ground stations had pm2.5 values as low as 9.5 g / m . below ground stations had generally much higher concentrations . 
 indeed ,
st ( e line ) , 34th st ( e line ) , 
 and 23rd st ( nqr line ) with means  sds of 35.1  8.5 , 40.8 
  21.7 , and 44.9  9.2 g / m , respectively . 

the stations that had the highest mean concentrations 
 were 14th st ( f line ) , west 4th st ( f line ) , and 34th st ( f line ) , 
 with means  sds of 200.4  91.6 , 156.7  47.8 , and 
 147  40.7 g / m , respectively .
the pm2.5 concentrations of all stations sampled during 
 the study period are provided in table 1 .
the 
 averaging time of each reading was 510 min , depending on train 
 arrivals .
the pm2.5 mean  sd concentrations for stations on the f , e , 
 and 6 subway lines were very elevated : 139.3  63.1 , 64.8  
 37.5 and 106.9  31.3 g / m , respectively .
the 
 concentrations of pm2.5 recorded at each station are given 
 in table 4 .
each value 
 reported is the average 
 of two mean measurements where each measurement was a mean of 510 
 min of sampling .
the first measurement was taken in the uptown direction , 
 and the second was when the student reached the same station again 
 in the downtown direction .
this station s concentration 
 is significantly different from the concentrations of the other two 
 stations located at the same cross street .
the roadside ambient pm2.5 concentration 
 measured aboveground 
 during the group sampling study was 9.8  4.8 g / m and much lower than the levels in the subway stations ( figure 3 ) .
pm2.5 concentrations at the roadside and in 
 stations 
 along the three parallel subway lines sampled during the group sampling 
 run .
the box shows the interquartile range , the line in the box shows 
 the median , and the number in the box shows the mean .
the whiskers 
 represent the upper 25% and lower 25% ( excluding outliers ) , while 
 the * symbol represents outliers .
in addition to the real time pdr 1500 dataram measurements for 
 pm2.5 , filter - based pm2.5 concentration at astor 
 place on two occasions was 232 and 448 g / m , respectively . 
 for comparison , the mean real time - based pm2.5 concentrations 
 were 114 and , 150 g / m , respectively .
trace 
 element analysis for one astor place filter showed that fe , mn , and 
 cr levels were high ( consistent with previous studies ) measuring 84 
 g / m , 693 ng / m , and 386 ng / m , respectively .
figure 4 shows the variation in bc2.5 and pm2.5 concentrations at stations along the number 7 subway line , 
 which has both underground and aboveground stations on the same subway 
 line .
the mean bc2.5 concentration of the underground subway 
 stations was 14.0  4.4 g / m , while the mean 
 concentration of the aboveground stations was 2.8  2.3 g / m ( each station s mean measurement being a 510 
 min average ) . similarly ,
the mean pm2.5 concentration at 
 the underground subway stations was much higher ( 93.3  30.5 
 g / m ) than at the aboveground stations ( 12.2  
 4.0 g / m ) .
time series graph showing the spatial change 
 in bc2.5 and pm2.5 at stations while traveling 
 from an underground 
 station to aboveground stations and then back to underground stations .
pm2.5 mass and bc / ec concentration levels have been 
 studied in subway stations in different parts of the world in cities 
 such as london , stockholm , helsinki , and amsterdam . however , this 
 study is the first time that bc levels have been measured at subway 
 stations in new york city . of
the various particles generated 
 during fossil fuel combustion , 
 particularly in diesel - generated exhaust , exposure to black carbon 
 ( bc ) , sometimes referred to as soot or black smoke , is known to be 
 associated with a range of adverse respiratory health effects and , 
 therefore , has been studied with increased interest in recent years . in some studies , the dark component of particulate matter known 
 as black carbon may also be reported as elemental carbon ( ec ) , which 
 is highly correlated but has a different method(s ) of detection . in 
 new york city , it has been found that diesel - generated elemental carbon 
 soot was the factor most closely associated with pollution - related 
 asthma exacerbation in children living near roadways and is strongly 
 associated with personal measures of ec .
a review of studies showing short - term effects of bc showed that bc can have a greater effect on 
 health compared with pm2.5 and pm10 .
although 
 this study was primarily focused on measuring airborne 
 particle concentrations on subway station platforms , bc and pm2.5 levels were also measured inside the train car during a 
 single commute and found to be approximately 1/4 to 1/3 the station 
 concentrations along the same line .
further analyses of in - car measurements 
 were not done in the study , although air conditioning was likely responsible 
 for the lower levels .
bc concentrations of the subway stations 
 ranged as low as 5 g / m although most stations 
 bc concentrations were in 
 the range of 1020 g / m .
it is important 
 to note here that nyc subway passenger trains are powered by electricity 
 and , thus , it is somewhat unexpected to find relatively high levels 
 of bc in the subway system , compared to ambient levels .
subsequent 
 investigations revealed that diesel - powered trains are used for maintenance 
 and operate during the night , which is one likely cause for the buildup 
 of bc levels in the subway system air . in the group sampling 

experiment along three parallel subway lines , 
 the bc2.5 concentrations in the underground subway stations 
 were several times higher than the ambient roadside concentration . 

this is consistent with other studies that have looked at bc in subway 
 stations and outside the subway system .
a recent study reported a mean of 9 g / m for 
 underground subway stations in amsterdam and rotterdam , which was 
 five times higher than aboveground levels . in the present study , the 
 bc2.5 concentrations in the stations sampled along the 
 selected f , e , and 6 subway lines were seven , four , and six times 
 higher than the ambient roadside bc2.5 concentrations , 
 respectively .
interestingly , the nyc underground f , e , and 6 subway 
 station bc2.5 means of 16.9 , 10.2 , and 15.3 g / m were higher than previously reported values for subways in 
 amsterdam and helsinki .
this suggests that nyc s 
 subway stations are more polluted in terms of bc2.5 levels 
 than other subway systems studied to - date . as discussed above , 
 concentrations of bc in nyc subway stations 
 measured in this study
therefore , it is an important 
 factor to consider in terms of potential health effects .
most research 
 on bc soot has looked into the effects of diesel exhaust , while measuring 
 bc levels as a component . a pooled analysis of the available studies have , however , shown 
 that a 1 g / m increase in ec is associated with 
 a 1.45% increase in all cause mortality and a 1.77% increase cardiovascular 
 mortality . in a study that looked at acute respiratory health among bronx schoolchildren 
 with asthma ,
adverse health effects were found to be strongly associated 
 with personal measures of ec exposure , suggesting the diesel soot 
 fraction of pm2.5 derived from high volume roadways was 
 most responsible for the pollution - related asthma exacerbations .
the study found that same day elevated risk 
 of wheeze ( 1.45 ) , shortness of breath ( 1.41 ) , and total symptoms ( 1.30 ) 
 were associated with an increase of personal ec exposure .
it is important 
 to note than in all of the above studies , the measured bc / ec levels 
 were between 1 and 5 g / m .
therefore , the much higher 
 mean concentrations found in nyc subway stations in the present study 
 could be considered as a health risk to individuals exposed to such 
 concentrations , especially in the long term .
however , the bc particles 
 measured in the subway system are re - entrained by the rapidly passing 
 trains , and not always freshly emitted , and the potential for bc soot 
 particles found in the subways to cause adverse health impacts may 
 also depend on whether the diesel particles are fresh or  aged  . 

fresh diesel exhaust has been shown to produce toxicity in various 
 organs and cells , but studies on aged particles , 
 such as were measured in this study , are limited .
some studies have 
 shown that both fresh and aged diesel exhaust particles have high 
 oxidant generation capacity and toxicity .
thus , the human health implications of these subway exposures to 
 apparently re - entrained aged diesel particles may or may not be the 
 same as diesel particle exposures in the ambient environment .
bc levels in the sampled nyc subway stations showed high spatial 
 variation depending on the station , depth of the station , and line 
 sampled .
measurements from the west 4th st station showed that the 
 bc2.5 concentrations in the upper level ( a , c , and e train 
 stop ) were significantly lower than in the lower level ( b , d , f , and 
 m train stop ) by approximately 40% .
significant variations in bc2.5 were seen among stations located along each line , possibly 
 due to the depth of the station , efficiency of ventilation , and number 
 of trains passing through the station .
it was observed that the stations 
 that generally had lower bc2.5 concentrations , such as 
 the 8th st nyu station , were closer to the road level and appeared 
 to have better ventilation .
the stations at lower levels , such as 
 the west 4th f train stop and 14th st f stop , recorded much higher 
 bc2.5 levels .
bc2.5 concentrations also 
 varied greatly between aboveground 
 and underground stations sampled along the 7 train line .
concentrations 
 were several times higher in the underground stations compared with 
 aboveground ( figure 4 ) .
the underground station 
 closest to an aboveground station had a lower bc2.5 concentration 
 than an underground station located further away from the aboveground 
 station .
thus , ventilation may play an important role in the concentration 
 of bc2.5 in underground subway stations .
the variation 
 in bc2.5 levels was also dependent on 
 possible sources within the subway system . during sampling along the 
 nqr subway line ,
the concentration of bc rapidly increased to more 
 than 100 g / m ( figure 2 ) 
 when a diesel - powered maintenance train passed through the station . 

this is evidence that the diesel - powered maintenance trains running 
 at nights could be a significant contributor to elevated bc2.5 levels within subway stations as well as throughout the subway system . 

these peak exposures may be important , in terms of health impacts , 
 for commuters and particularly for subway workers who work at night 
 and are part of the maintenance crew , since they may be exposed to 
 higher bc2.5 levels during a greater portion of their work 
 shift .
in addition to real time measurements of bc2.5 in subway 
 stations , filter - based measurements of ec2.5 were obtained 
 using quartz filters .
although the real time aethalometer and the 
 filter sampling occurred during the same time interval on only a limited 
 number of samples ( n = 3 ) , the comparison of the 
 two methods showed that the real time mean bc2.5 measurements 
 were approximately 6080% higher than the filter - based mean 
 ec measurement at the astor place station and 20% higher along the 
 6 line .
possible causes for this difference in bc and ec concentrations 
 could be the presence of fine particles that are  dark  , 
 differences in measuring methods , and light absorption by metals . 

however , a 2.5 m cut cyclone was used with the aethalometer 
 to minimize possible confounding of real time measurements from larger 
 particles that are  dark / black  .
the difference 
 in bc and ec measurements can be somewhat expected 
 since the basis used for the measurements in each method are different . 

aethalometers measure in real time utilizing light scattering and 
 absorbance , while the measurement of ec by the niosh method uses thermal 
 methods .
as reviewed in a study , bc and 
 ec measurements do not always agree and calibration between the two 
 methods is needed for each site / environment to have a better understanding 
 of the relationship of the two components and methods .
regardless , 
 ec measurements were also between 1012.5 g / m in the stations and lines measured , thereby confirming that concentrations 
 of the diesel soot component in the subway stations is several times 
 higher than roadside levels . a study conducted in the berlin subway , 
 where diesel engines are used , reported mean ec levels of 10.9 and 
 6.9 g / m in summer and winter , respectively .
real - time measurement of pm2.5 in nyc subway stations 
 ranged from 35200 g / m , which , as with the 
 bc2.5 measurements , was several times higher than the ambient 
 roadside mean concentration of 9.5 g / m .
similar 
 results have been shown in other studies that have examined pm2.5 concentrations in subway systems .
in nyc s subways , 
 an earlier study reported a mean pm2.5 concentration of 62 g / m .
studies done 
 on nyc subway workers showed various effects on selected biomarkers 
 in workers , in particular , higher concentrations of chromium in plasma 
 and dna cross - links , although the chromium levels were below occupational 
 standards . in studies in london ,
stockholm , 
 and helsinki , mean pm2.5 concentrations in the subway were 
 247.2 and 157.3 g / m in london during two seasons , 
 260 g / m in stockholm , and 60 g / m in helsinki , levels which were 38 , 510 , and 45 
 times higher than ambient levels measured in busy urban areas in their 
 respective studies .
therefore , 
 the present study provides further evidence that pm2.5 levels , 
 and thus their sources , in underground subway systems are several 
 times higher than busy urban roadside levels .
ambient roadside measurements 
 and subway station measurements were done during the same sampling 
 run to minimize errors and variations due to time / day and seasons . 

pm2.5 levels showed a similar pattern to bc in the aboveground 
 vs underground sampling , which was measured simultaneously to bc measurements 
 ( figure 4 ) .
although pm2.5 levels 
 were several times higher than bc levels in this study , they were 
 highly correlated which suggests that both bc and pm2.5 may contribute to adverse health effects in commuters and subway 
 workers .
concentrations of pm2.5 at a single subway 
 station were 
 also obtained twice for gravimetric analysis .
the gravimetric pm2.5 concentrations at the astor place station were 232 and 
 448 g / m , indicating that the actual pm2.5 concentrations were 2 to 3-fold higher than those assessed with 
 the real time monitor ( for the subway environment under the same conditions ) . 

this variability was likely due to the fact that the real time pdr 
 1500 instruments had been factory calibrated to standard dust , whereas 
 the pm composition of the nyc subway has a higher proportion of metal 
 particles such as fe , mn , and cr , as shown by previous studies .
it 
 is also important to note that the pdr 1500 units were used with humidity 
 correction , which may have also contributed to the difference .
because subway commuters and workers spend the majority of their 
 day outside of the subway system , it is important to consider the 
 contribution of underground subway exposures to a total daily exposure . 

we can estimate that , the high levels of pm2.5 in the subways , 
 for example , can contribute to an estimated 30% higher daily pm2.5 exposure for subway commuters .
this calculation assumes 
 an ambient pm2.5 concentration of 10 g / m and that a commuter waits for a total of 30 min , round trip , in 
 subway stations along the f subway line ( observed to have the highest 
 pollution concentrations ) at an exposure concentration of 150 g / m pm2.5 in the subway .
importantly , this augmented 
 particle burden increases from 240 to 1200 g / mh , 
 assuming an 8 h underground work shift in the subway system . in this 

context , acute and chronic health effects of bc and pm2.5 may be more relevant for workers and vendors who spend a significantly 
 greater amount of time underground in subways .
while our data 
 demonstrate significant spatial variation in pm 
 and bc concentrations ( e.g. , upper vs lower stations ; f line vs other 
 lines ) , sampling at some stations were limited and greater uncertainty 
 may be present in such situations .
the study had some limitations , 
 partly due to subway security concerns 
 that were pertinent during the time of sampling .
the equipment that 
 could be carried in the subways and the durations they could be operated 
 were limited , as passengers who may not understand the procedure would 
 see it as a cause of concern .
this required sampling at only certain 
 stations in selected lines and limited the sampling time in the subway 
 stations .
filter - based sampling required at least 1 h of sample collection 
 at each station , which was not possible given the security concerns 
 at that time .
there were also limitations with the group sampling 
 experiment 
 in maintaining the same time interval between stations at the same 
 cross streets , due to train arrival times and delays ; although the 
 times were approximately within the same 510 min time period . in conclusion
, this is the first to demonstrate elevated bc2.5 concentrations in nyc s subway stations , showing 
 diesel exhaust could be present in the subway system .
the pm2.5 and bc2.5 levels in the subway stations were found to 
 be several times higher compared to roadside ambient levels and aboveground 
 subway station levels .
pm2.5 and bc2.5 concentrations 
 showed considerable variability among stations , lines , level of station , 
 and location of stations .
seasonal variations were not seen at the 
 single station sampled several times during both summer and winter 
 ( data not shown ) .
the study also shows that bc2.5 levels 
 increase rapidly when a diesel maintenance train is in operation locally , 
 thereby being a potentially important contributor to elevated bc2.5 levels in subway stations .
overall , the pm2.5 and bc2.5 concentrations in nyc subway stations were 
 higher than previously reported levels in subway systems in europe 
 and , importantly , at pm2.5 levels shown to be associated 
 with adverse health effects .
these levels could therefore have significant 
 health impacts on commuters but would likely be of the greatest health 
 import to subway workers , who spend much longer times on subway station 
 platforms and diesel - powered maintenance trains .
diesel engines can 
 have particulate matter traps applied to their exhaust to reduce emissions , 
 providing one potential exposure mitigation measure .
therefore , further 
 air pollution exposure and health effects studies are warranted for 
 underground subway lines and stations , focusing on identifying the 
 contributors to higher levels of pm2.5 and bc2.5 in the subways of nyc , assessment of the potential health effects 
 of ongoing exposure to passengers and workers , and a determination 
 of potential mitigation measures available ( such as improved fresh 
 air ventilation , subway air filtration , or diesel train emission controls ) .
 the 
 new york city ( nyc ) subway is the main mode of transport for 
 over 5 million passengers on an average weekday . 
 therefore , airborne 
 pollutants in the subway stations could have a significant impact 
 on commuters and subway workers . 
 this study looked at black carbon 
 ( bc ) and particulate matter ( pm2.5 ) concentrations in selected 
 subway stations in manhattan . 
 bc and pm2.5 levels were 
 measured in real time using a micro - aethalometer and a pdr-1500 dataram , 
 respectively . 
 simultaneous samples were also collected on quartz filters 
 for organic and elemental carbon ( oc / ec ) analysis and on teflon filters 
 for gravimetric and trace element analysis . in the underground subway 
 stations , 
 mean real time bc concentrations ranged from 5 to 23 g / m3 , with 1 min average peaks > 100 g / m3 , 
 while 
 real time pm2.5 levels ranged from 35 to 200 g / m3 . 
 mean ec levels ranged from 9 to 12.5 g / m3 . at street level on the same days , 
 the mean bc and pm2.5 concentrations were below 3 and 10 g / m3 , respectively . 
 
 this study shows that both bc soot and pm levels in nyc s subways 
 are considerably higher than ambient urban street levels and that 
 further monitoring and investigation of bc and pm subway exposures 
 are warranted . 
Introduction
Materials and Methods
Results
Discussion