we carried out a prospective , unblinded study in six healthy volunteers at the prince of wales hospital in shatin , hong kong .
we compared the filtration capacity of the stryker t4 and stackhouse freedomaire surgical helmets with an 8233 n100 filtering facepiece respirator ( 3 m , st .
approval was obtained from the clinical research ethics committee of the chinese university of hong kong .
each test measured the ability of the device to filter ambient dust particles , normally present in room air , by using a previously described standard , quantitative , fit - testing protocol ( 9 ) . in brief , the testing compared particle counts inside and outside the protective device during a series of activities  normal breathing , deep breathing , turning the head from side to side , flexing and extending the head , talking loudly , and bending over followed by normal breathing .
the tube for sampling the mask particle count was connected to a test probe designed for this purpose ( tsi incorporated , st .
paul , mn ) , which was inserted through the fabric of the protective device . on the n100 respirator
, the probe was passed through both the respirator and covering surgical mask 1 cm to the right of the valve .
on the surgical helmets , the probe was placed centrally in the breathing zone 1 cm below the bottom edge of the transparent face piece .
the tube for sampling the ambient particle count was fixed approximately 3 cm from the sampling probe .
no participant had been previously fit tested on this brand of n100 respirator ; however , all participants received instructions on donning both the respirators and the surgical helmets before use . before each test we checked that all participants were wearing their devices correctly .
a portacount plus ( tsi incorporated ) connected to a computer running fitplus for windows software ( tsi incorporated ) was used to count particles and calculate the ratio of ambient - to - device particle counts .
this device counts all particles between 0.02 and 1 m in diameter ; it also calculates a fit factor , which is the average ratio of ambient - to - device particle concentrations .
the equation used is   where : n is the number of exercises performed and ffj is the fit factor for the individual exercise .
the reusable tubing supplied by the manufacturer was replaced with disposable polyvinyl chloride ( pvc ) tubing of the same internal diameter and length to minimize any risk for cross - infection . to ensure an adequate ambient particle count , the 8026 particle generator ( tsi incorported )
when the surgical helmet - hood combinations were being tested , the helmet and hood were put on and then a disposable surgical gown ( microcool specialty gown , kimberly - clark , roswell , ga ) was worn over the top of the hood , in accordance with the manufacturer s instructions . since buildup of carbon dioxide has been found to be a problem with these helmets ( 10 ) , the highest fan speed was used throughout the testing . during testing of the n100 mask ,
the participants wore a standard three - ply surgical mask ( surgicos johnson & johnson , arlington , tx ) tied over the top ( since the n100 mask is not licensed for use as a surgical mask ) and a full face shield ( splash shield , woburn , ma ) .
the median ratios of ambient - to - device particle counts were compared by using the mann - whitney u test ( statview 5.0 , sas institute , cary , nc ) .
during the tests , the median ambient concentration of 0.02- to 1-m particles was 7,650/cm ( range 3,98029,200/cm ) .
results of the filtration capacity of the three devices are shown in the table . in all tests ,
the n100 mask filtered significantly more particles than either of the surgical helmet - hoods . during testing , a half - face respirator , such as the n100 mask , should reduce the particle count by a minimum of a factor of 100 ( 11 ) .
the greatest particle count reduction achieved with a surgical helmet - hood was a factor of 4.8 .
ratios for stryker t4 and stackhouse freedomaire were significantly lower in all tests compared to the combination of n100 mask , surgical mask , and face shield ( p < 0.004 . )
our data demonstrate that both surgical helmet - hoods have markedly inferior in vivo filtration performance compared to the combination of n100 mask , surgical mask , and face shield .
more importantly , both surgical helmet - hoods failed in all cases to meet the national institute for occupational safety and health performance requirement for even a half - mask respirator .
clearly , this failure rate would be unacceptable if these devices were to be considered for use as respirators .
neither surgical helmet is approved as a respirator nor marketed as a method of protecting the user against respiratory pathogens .
in fact , stryker recommends that its helmet be used in combination with additional eye and respiratory protection in this setting ( available from : url : http://sars.medtau.org/strykerreport.doc ) .
first , we tested filtration of particles , not the coronavirus postulated to cause sars .
in addition , it is impossible to be certain what size of particles the surgical helmet - hoods were failing to adequately filter , nor is it obvious which particle size is most important to filter , since many aerosolized particles will be larger than a naked coronavirus .
it is therefore conceivable , but we believe unlikely , that the surgical helmet - hoods would efficiently filter coronavirus - containing particles .
second , we modified the portacount plus by using disposable tubing rather than reusable tubing .
as the disposable tubing and the tubing supplied by the manufacturer are both pvc , and of the same internal diameter and length , this change is unlikely to have made a difference in the results .
sars is believed to be transmitted by contact of the virus with mucosal surfaces such as the eyes , as is the case with other respiratory viruses such as respiratory syncytial virus ( 12 ) .
although both surgical helmet - hoods reduce the particle count compared to ambient counts , we believe this benefit may be counteracted by the fact that both devices direct a flow of gas into the eyes .
finally , the high particle count inside the hoods might have been due to the fan s blowing particles off the hood material , the wearer s head , or even the fan itself . in further experimentation
, we found that when the surgical helmet was worn inside a papr system , the particle count inside the helmet was low , regardless of whether the fan was turned on or off ( j. l. derrick & c.d .
particles might also be drawn up from under the hood rather than through the hood material . in this case , the exact mechanism of entry would be irrelevant , as in both cases the indrawn air would be potentially contaminated if the patient had sars .
our data indicate that neither the stryker t4 nor the stackhouse freedomaire helmet - hood filters enough particles of 0.021 m in diameter to meet the standard for protective respirators . as the size of coronaviruses falls within this range ,
 performance testing of two brands of surgical helmets indicated that their efficiency at in vivo filtration of sub  micrometer - sized particles is inadequate for their use as respirators . 
 these helmets are not marketed for respiratory protection and should not be used alone for protection against severe acute respiratory syndrome when performing aerosol - generating procedures . 
Materials and Methods
Results
Discussion