A. Johnson, K. Mackey, W. Scott, F. Koh, University of Maryland, College Park, MD.
A tight-fitting PAPR supplies inhalation air through a filter, but the filter resistance is not usually detectable by the user. Instead, the fan is used to overcome the filter resistance. If the fan cannot supply as much air as required by the working wearer, then the filter resistance is partially imposed upon the wearer. The full filter resistance burden should be levied at breathing flow rates exceeding the fan flow rate. Previous research results have shown that work performance time decreases linearly with increased resistance. The effect of filter resistance on performance time is not known when the filter is felt for only part of the breathing cycle. It might be expected that, as breathing flow rates exceed fan flow rates by greater amounts, performance time would decrease. Subjects walked on a treadmill at 80–85% VO2max until volitional fatigue while wearing a tight-fitting PAPR. The voltage on the PAPR fan was varied in increments of 25%, from 0 to 100% of rated voltage for different test sessions. Fan flow rates at these voltages were determined from headform tests. The no-voltage condition should be equivalent to breathing through a filter with a slightly higher resistance due to the presence of the passive fan. Results show a very low sensitivity of performance time to fan voltage level, with no systematic relationship between voltage level and performance time.
A. Johnson, K. Mackey, W. Scott, F. Koh, University of Maryland, College Park, MD.
Loose-fitting PAPR are comfortable solutions to the problem of exclusion from dust and other airborne contaminants. A multipurpose PAPR has been proposed for the mining industry to protect respiration, head impact, hearing, and vision. When working strenuously, breathing flow rates increase greatly, and can exceed the capacity of the fan (140 Lpm) to supply filtered air. This study was designed to determine how much unfiltered air a person might be expected to breathe. Subjects walked on a treadmill at 80–85% of VO2max. They wore the PAPR, over which was a portable breathing chamber (PBC) consisting of a closed polyethylene container with an inlet through which air could be drawn by the fan and an inlet/outlet through which air could move to or from the surroundings. The inlet/outlet was fitted with a pneumotach to measure flow rates, and the PBC was sealed with tape around the neck of the subject. Inhaled air through the pneumotach indicated that the subject needed more air than was supplied by the PAPR fan. Data were sampled at 50 Hz. Results showed that overbreathing routinely occurred. Overbreathing flow rates of up to 300 L/min were recorded in one subject, although peak overbreathing flows of about 100L/min were more typical. The significance of these flows depends upon the time for which they are sustained and the volume of the filtered air inside the face shield. High flow rates sustained for short times do not result in breathing contaminated air, but those sustained for a long time can result in contaminant exposure.
A. Johnson, K. Mackey, W. Scott, F. Koh, University of Maryland, College Park, MD.
Respirators are worn by workers when engineering controls to alleviate airborne contamination cannot be realistically implemented. Respirator filters are tested and certified at 85 L/min, which is adequate for light and moderate work, but perhaps too low for high work rates. Characterization of the flow rate distributions of people breathing at high work rates needs to be accomplished in order to guide future testing scenarios. Twelve subjects walked on a treadmill at an intense work rate of 80–85% of VO2max. Flow rates were measured in two ways: (1) by a low-resistance Fleisch #4 pneumotach (resistance of 0.067 cm H2O.sec/L), and (2) by the pressure drop across the low resistance filter of an SEA 400AT demand PAPR. Flow rates were sampled at 50 Hz and flow rate distributions were plotted. The lower flow rates (0–20 L/min) occupied larger percentages of the flow rate histogram, and the percentages generally decreased as flow rates increased. Maximum recorded flow rates generally were below 240 L/min, with some as high as 600 L/min when measured with the pneumotach. Peak flow rates measured with the PAPR were considerably higher, generally about 500 L/min. The reasons for the differences are not entirely known. The importance of these measurements depends upon the nonlinearity of the filter pressure-flow curve. If the filter pressure drop is linearly related to flow rate, then the flow rate at which it is tested is of no consequence. If the curve is highly nonlinear, then several representative flow rates should be used
L. Janssen, 3M Company, St. Paul, MN.
Airflow through an orifice is known to increase along with pressure drop across that orifice. It has been proposed that if this phenomenon occurs with respirator faceseal leaks, an increase in pressure drop (breathing resistance) would increase airflow through the leak. It has been further suggested that respirator user exposure to contaminants could increase because of this increased airflow. Procedures similar to those used by previous investigators were modified to study this issue. A controlled negative pressure quantitative fit test instrument was used to make repeated faceseal leak rate measurements at 5.6 through 20.1 mm water pressure drops across the faceseal. Subjects were divided into two groups, representing acceptable fit or unacceptable fit, based on leak rate criteria prescribed by the Occupational Safety and Health Administration. Subjects with adequate fit did not experience an increase in leak rate with increased pressure drop. Leak rates for subjects with inadequate fit were highly variable and did not show an association with pressure drop. Results of this study do not support the concept of increased faceseal leakage with increased pressure drop. The evidence does not suggest increased risk of contaminant exposure through the faceseal as pressure drop increases.
T. Reponen, S. Lee, A. Adhikari, R. McKay, S. Grinshpun, University of Cincinnati, Cincinnati, OH.
Agricultural workers are frequently exposed to high concentrations of airborne fungal spores. Very little information is available on the protection provided by respirators against fungal spores and other particles in agricultural workplaces. We have developed a setup for determining the efficiency of respirators directly on a human subject. The setup allows simultaneous collecting of fungal spores and non-biological particles inside and outside N95 filtering facepiece respirators. It includes two identical sampling lines, each consisting of a sampling probe, Tygon tubing, a sampling chamber, an optical particle counter (measurement range 0.7–10 µm), a 25-mm cassette with a 3-µm polycarbonate filter for fungal spore sampling, and a 10 L/min personal sampling pump. In this study, we tested the new method in the laboratory (with NaCl test particles) and in the field (in a swine and a poultry farm during animal feeding). The protection factor, PF, was determined by dividing the number concentration of particles outside the respirator by the number concentration inside the respirator. The laboratory tests were performed using a manikin with artificially created leakages. Increasing the total leak area from 2 to 8 mm2 decreased the PF from 25 to 10 when the total airflow through the respirator was 50 L/min. Increasing the flow rate from 10 to 95 L/min decreased the PF from 13 to 6 when the total leak area was held constant at 8 mm2. The field-testing with human subjects showed PF values between 19 and 109 for the total concentration of particles. The PF values varied with particle size, increasing with an increase in the optical particle diameter. The PF for culturable fungal spores ranged from 1.2 to >96 and was generally lower than that determined for the total dust in the respective particle size range.
Z. Frund, MSA Co., Cranberry Township, PA.
During the past year, the National Institute of Occupational Safety and Health has released standards for the certification of air-purifying respirators (APRs) to be used by individuals (a) responding to incidents associated with weapons of mass destruction (WMDs), and (b) attempting to escape from areas having WMDs. Analysis of the many comments regarding the use and limitations of such devices from first responders and others performing rescue and cleanup-related tasks at the WTC and Pentagon sites served as the basis for the standards criteria. The standards quantify, among other things, the minimum required effectiveness that APRs must have against chemical warfare agents and a broad range of toxic industrial compounds/toxic industrial materials (which may also be present during a terrorist event).
Respirator manufacturers, in response to the newly adopted performance criteria, have developed devices meeting the appropriate certification standards. This presentation will address: extensive laboratory testing and technical hurdles overcome by respirator manufacturers to meet the certification standards; laboratory results for APRs when tested against the newly adopted standards; examples of APRs meeting the certification standards; factors to consider when selecting and using respirators when a potential for exposure to WMDs exists; and the future direction of respiratory devices for protection against WMDs.
Posted May 30, 2004