Quantifying Exposure Risk; Surgical Masks and Respirators

Background: At present there are limited data to support current CDC recommendations for the use of N95 respirators to prevent H1N1 infection of healthcare workers (HCW).  The current literature focuses on inhalational barrier protection (filtration) worn by the HCW.  Reducing exposure at the source has not been quantified in terms of relative protection of the HCW utilizing current NIOSH workplace protection factors. 

Objective: Using a bench model we measured the effects of filtration, dilution and deflection of potentially infectious particles achieved by surgical masks and N95 respirators.

Methods: To quantify exposure we constructed a chamber designed to produce radiolabeled wet aerosols simulating contaminated particles exhaled during tidal breathing (Source). Source aerosols were exhaled via a ventilated mannequin head (Simulaids, Saugerties, NY) suitable for mask protection.  A similar ventilated head was placed within the chamber to assess recipient exposure (Receiver).  Aerosols were created by an AeroTech II nebulizer (Biodex, Shirley, NY).  Expelled particles near the source and near the receiver were measured by cascade impaction.  Exposure was quantified by placing a filter within the receiver head.  We tested two types of masks, a NIOSH approved N95 respirator (3M, St. Paul, MN) and an earloop surgical mask (Crosstex International Inc., Hauppauge, NY).  Each head was connected to a Harvard pump (Millis, MA), tidal volume 500ml, respiratory rate of 15, and duty cycle of 0.5.  Several mask combinations were assessed: no mask, a surgical mask tightly fit on the source (STSM), a surgical mask loosely fit on the source (SLSM) and an N95 on the receiver (RN95) or on the source (SN95).  Data were presented as a percent of total aerosol on the receiver filter and the ratio of maximum exposure to actual exposure was calculated as a Workplace Protection Factor (WPF).

Results: See Table

Conclusions: Our model consistently reproduces aerosols comparable to the HCW work environment.  It was observed that wet aerosols emitted from a potentially infectious source evaporate to sub-micron particles.  Mask filtration when applied either at the source or the recipient, does not play a significant role in preventing those particles from reaching the recipient. Deflection and dilution of exhaled particles, such as can be achieved with a surgical mask worn at the source, achieve far greater levels of protection than achieved through the use of an N95 respirator on the recipient.

S Mask	R Mask	RMMAD	% Inhaled	% Mask	WPF
None	None	0.63µm	1.4	N/A	N/A
None	N95	0.63µm	0.79	0.13	1.74
TSM	None	0.46µm	0.0038	13	366
N95	None	ND	0.0031	40	442
LSM	None	0.48µm	0.0042	6.0	328
LSM	N95	0.48µm	0.0060	6.0/0.0042	366

S = Source
R = Receiver
TSM = a tightly fit surgical mask
LSM = a loosely fit surgical mask
R MMAD = mass median aerodynamic diameter at the Receiver
S MMAD = 1.05μm
% Inhaled = % of total aerosol reaching the receiver
% Mask = % of total aerosol captured by mask
WPF = Workplace Protection Factor

ND = Not Done