The Physiological Effects of Respiratory Protection in an Australian Smelter

Respiratory protection is commonly used to mitigate worker exposures to airborne contaminants. Wearing a respirator creates an external thoracic obstruction on wearer, changing how people breathe which in turn modifies the breathing pattern. That is, it attenuates the minute ventilation and increases the time required for inhalation compared to exhalation. The respirator alters the shape of the flow-volume curve, creating a respiratory strain response for the wearer. Respirator use places both physiological and psychophysiological strain on the individual depending on the type of respirator, the work environment, and the individual worker’s work rate.

The physiological and psychophysiological effects on respirator wearers have recently been recognised by the International Standards Organisation (ISO) introducing Human Factors into the design and testing of respiratory protection via their 16900 series of standards. Australian and New Zealand Standards Organisations have adopted these ISO standards with a 5-year phase out of their current standards. The ISO standards set respirator selection criteria based on work rate and nominal respiratory rates informed by previous laboratory studies. Due to difficulties with measuring in-respirator respiratory strain responses, no in-workplace studies have been published to underpin, validate, or test these standards.

The research reported in this thesis presents three novel stages of enquiry to “identify real- time respiratory rates for respirator wearers in a workplace and determine the physiological and psychophysiological effects of respirator use on the workers who depend on them”. A series of studies were conducted and evaluated using a modified stimulus response model developed to evaluate the physiological and psychophysiological responses to respirator use, determining the suitable instrumentation and validating its use, eventually leading to testing the real time effects of respirator use in a real workplace- a smelter environment in far north Queensland.

Once the modified stimulus-response model was developed (Chapter 1), a review of measurement devices suitable for in workplace respiratory measurement was conducted and the application of the novel S.E.A. pressure data logger (PDL) was explored to determine its suitability for in-workplace measurements of respiratory strain responses without altering the mechanism of the respirator (Chapter 2).

The precision, accuracy, calibration, and use of the PDL was evaluated and validated against the industry standard Cortex METAMAX^® 3B and a calibrated breathing simulator and it was found acceptable for use (Chapter 3). It was then deployed using seasoned industry respirator wearers (smelter workers) (N=18) in a unique random cross over laboratory trial at varying work rates in a laboratory study under controlled conditions with heart rate as an anchor (Chapter 4). The effect of the respirator was isolated from other environmental stressors, and consistent with previous laboratory studies, a significant decrease in minute ventilation (V̇_I), peak inspiratory flow (PIF) and breathing frequency (F_b) was observed with respirator use versus no respirator. There was also a significant increase in the time of inhalation (t_I) over the time of the respiratory cycle (t_I/t_TOT) indicating that the shape of the respiratory curve had changed from sinusoidal to elliptical. Although heart rate and ratings of perceived dyspnoea (RPD) increased with respirator use and with work rate; they were not significantly different with the respirator compared to without.

Following successful deployment in a controlled laboratory situation, the PDL was then utilised with the same cohort (N=38) in a smelter workplace environment in a first of its kind study designed to examine cardiorespiratory, thermoregulatory, and psychophysiological strain responses to respirator use (Chapter 5). A within participant (work area) and between conditions (work tasks with requiring different work rates) analysis was conducted through exposure to a range of uncontrolled stressors i.e., different work environments and varying work rates. Although condensation in the equipment lines and drift due to the harsh environment led to some loss of data, the novel S.E.A. PDL was successfully deployed for in-respirator and in-workplace measurement of respiratory parameters across a range of activities in a harsh environment. Thus, demonstrating the PDL to be a useful and valid instrument for measurement of in-respirator in-workplace respiratory strain responses.

As expected, V̇_I, HR, PIF, F_b and RPD increased with higher work rates and all activities were significantly different when compared to resting. There was a reduction in t_I leading to higher t_I/t_TOT and an increase in RPD due to a change in the shape of the breathing curve at higher work rates. The primary aim of this research was achieved with the first in-workplace cardiorespiratory, thermoregulatory, and psychophysiological strain responses to wearing respiratory protection at different work rates in a smelter environment were reported and found physiologically acceptable for this cohort.

The Standards Australia/Standards New Zealand adoption of the ISO16900 series of standards on respiratory protection has significant implications for practice regarding the selection of respiratory protection for a work rate class (WRC) based on V̇_I. Given the results of the in-workplace evaluation, the challenges for the OHS Practitioner in selection of respiratory protection was then explored (Chapter 6).

Comparison of respirator selection approaches using estimated work rate compared to in-respirator respiratory measurements revealed that heart rate (HR) and rate of perceived dyspnoea (RPD) were adequate predictors of V̇_I for this cohort and thus could be used in respirator selection. However, direct measurement of V̇_I is recommended for each specific situation and individual wherever possible; and the S.E.A. PDL was demonstrated to be a useful and valid instrument for this purpose. Even then, caution must be exercised as it is possible that individuals will experience differences in the work of breathing (WOB) at the equivalent V̇_I. Therefore, it is essential that the individual is placed at the centre of any respiratory protection program as differences in cardiorespiratory fitness will determine the physiological strain responses to workplace stressors and the acceptability of specific respiratory protection.

The Australian Standards 5-year implementation phase for the 16900 series on respiratory protection will allow industry and OHS Practitioners some time to assimilate the changes and consider how to inform identification of work rate classes for the selection of respiratory protection. Further workplace studies of the novel kind presented in this thesis will build up a database of in-workplace data to inform those decisions, thus providing workers with physiologically acceptable respiratory protection that they can comfortably wear to protect against exposure to hazardous airborne contaminants.