To understand more about TWA and STEL Limits in Australian workplaces, please read the below extract from Safe Work Australia
DATE OF EFFECT: 18 April 2013
This document contains a list of workplace exposure standards for airborne contaminants (exposure standards) and how to meet your duties under the Work Health and Safety (WHS) Act and the WHS Regulations. In providing guidance, the word ‘should’ is used in this document to indicate a recommended course of action, while ‘may’ is used to indicate an optional course of action. This document also includes references to sections of the WHS Act and Regulations which set out the legal requirements. These references are not exhaustive. The words ‘must’, ‘requires’ or ‘mandatory’ indicate a legal requirement exists and must be complied with for you to comply with your duties under the WHS Act and Regulations.
6 2. INTERPRETATION OF EXPOSURE STANDARDS – Page Number: 6
2.1 The meaning of key terms
Airborne contaminant means a contaminant in the form of a fume, mist, gas, vapour or dust, and includes microorganisms. An airborne contaminant of this type is a potentially harmful substance that is either not naturally in the air or is present in an unnaturally high concentration and to which workers may be exposed in their working environment.
Breathing zone means a hemisphere of 300 mm radius extending in front of a person’s face and measured from the midpoint of an imaginary line joining the ears.
Exposure standard means an exposure standard in the Workplace Exposure Standard for Airborne Contaminants in Appendix A. An exposure standard listed in Appendix A represents the airborne concentration of a particular substance or mixture that must not be exceeded. The exposure standard can be of three forms:
a) 8-hour time-weighted average,
b) peak limitation, and
c) short term exposure limit.
Peak limitation means a maximum or peak airborne concentration of a substance determined over the shortest analytically practicable period of time which does not exceed 15 minutes.
Short term exposure limit (STEL) means the time-weighted average maximum airborne concentration of a substance calculated over a 15 minute period.
8-hour Time-weighted average (TWA) means the maximum average airborne concentration of a substance when calculated over an eight-hour working day, for a five-day working week.
2.2 Adjustment of exposure standards
To comply with the general duties under the WHS Act and specific duties in the WHS Regulations, the following issues should be taken into account when interpreting exposure standards.
Adjustment of 8-hour Time Weighted Average exposure standards
Where workers have a working day longer than eight hours or work more than 40 hours a week, the person conducting the business or undertaking must determine whether the TWA exposure standard needs to be adjusted to compensate for the greater exposure during the longer work shift, and decreased recovery time between shifts.
Peak limitation or Short Term Exposure Limit exposure standards must not be adjusted. 8-Hour TWA exposure standards must not be adjusted (increased) for shorter work shifts.
2.3 Other factors affecting risk
Not all chemical substances behave the same and therefore some present higher risks to workers than others. Factors that increase risks to workers must be considered when managing risks in the workplace to comply with duties under the WHS Act and Regulations to ensure, so far as is reasonably practicable, the health and safety of workers.7
Known factors that can increase risks to workers include:
Some substances easily penetrate intact skin and are absorbed into the body. Skin absorption may be a significant source of exposure. These substances are given the notation ‘Sk’ in column (5) of Appendix A.
Some substances are known to cause sensitisation and create greater risks to sensitised workers. These substances are given the notation ’Sen’ in column (5) of Appendix A. Sensitised workers may also react to levels of the substance below the exposure standard and should not be exposed further to the substance.
Read the Full Document