Hazard Ratio (HR) / Required minimum protection factor (RMPF)

A hazard ratio (HR) is measured airborne concentration of a substance divided by the occupational exposure limit (OEL)/Workplace Exposure Standards (WES)/Workplace Exposure Limit (WEL). To calculate the HR, it is necessary to know the foreseeable worst-case concentration(s) of the airborne contaminant(s) and the relevant occupational health regulatory requirements, exposure limits or the safe exposure levels applying to these contaminant(s). RPD with a protection level greater than the hazard ratio should be selected in order to reduce the wearer’s exposure to below the OEL.

Required minimum protection factor (RMPF) is the minimum protection factor required to reduce exposure to the OEL/WES/WEL. It is expressed as a ratio of the measured ambient airborne concentration of a contaminant to an acceptable exposure level or standard which is the same as determining the hazard ratio. This reduction is the ratio between the concentration of a contaminant outside the respirator to the concentration inside the respirator, i.e. breathed by the wearer. The required minimum protection factor (RMPF) for any given situation is that factor necessary to reduce the exposure of the wearer below an accepted level or exposure standard or to minimize the potential exposure. If the exposure standard of a substance is designated as a peak limit, then measurement should be taken of the peak ambient concentration. Similarly, if the exposure standard is a time-weighted average, measurements should be taken to estimate the eight-hour exposure level.

Example: Ambient workplace air sampling indicates airborne concentrations for chemical X ranging between 6 mg/m3 and 8 mg/m3. The OEL/WES/WEL for chemical X is 1 mg/m3. To calculate the HR/RMPF, one should use the highest concentration; in this case, 8 mg/m3. Using this formula an HR/RMPF of 8 is calculated (8 mg/m3 divided by 1 mg/m3), and therefore, an RPD that has a protection level of 8 or greater should be selected.

Reference/s for further information and detail:

  • AS/NZS 1715:2009 – Selection, use and maintenance – Section 4.2.2.2
  • SA/SNZ TS ISO 16975.1:2023 – Selection, use and maintenance – Section 7.3.2.2 and Annex C