What are exposure standards?
Exposure standards, listed in the Workplace Exposure Standards for Airborne Contaminants, represent the airborne concentration of a particular substance or mixture that must not be exceeded.
There are three types of exposure standard:
- 8-hour time-weighted average (TWA)
- short term exposure limit (STEL).
- peak limitation
8-HOUR TIME WEIGHTED AVERAGE EXPOSURE STANDARD
An eight-hour time-weighted average exposure standard is the average airborne concentration of a particular substance permitted over an eight-hour working day and a 5-day working week. These are the most common types of exposure standards.
SHORT TERM EXPOSURE LIMIT (STEL)
A short term exposure limit is the time-weighted maximum average airborne concentration of a particular substance permitted over a 15 minute period.
A STEL should not be exceeded at any time during a working day even if the eight-hour TWA average is within the TWA exposure standard. Exposures at the STEL should not be longer than 15 minutes and not be repeated more than four times per day. There should be at least 60 minutes between successive exposures at the STEL.
PEAK LIMITATION
Peak limitation exposure standards are a maximum or peak airborne concentration of a particular substance determined over the shortest analytically practicable period which does not exceed 15 minutes. A Peak limitation exposure standard must not be exceeded at any time.
Globally, countries are reviewing permissible gas standards with the objective to lower many of the concentrations.
In Australia, these are just some of the gases under consideration to reduce the TWA allowable limits:
- Ammonia from 25ppm to 20ppm
- Carbon Monoxide from 30ppm to 20ppm
- Chlorine from 1ppm to 0.1ppm
- Hydrogen Sulphide from 10ppm to 1ppm
- Hydrogen Cyanide from 10ppm (peak) to 0.9ppm (peak)
- Nitrogen Dioxide from 3ppm to 0.2ppm
- Nitric Oxide from 25ppm to 2ppm
No one would argue with the health benefits of lowering permissible levels in order to reduce the potential negative health impacts of these gases. But…
Can these gases be measuring accurately down to these concentrations?
Can calibration gas mixtures be manufactured down to these concentrations?
Most of these gases are commonly found in a variety of industries from food processing, water/wastewater, mining and oil/gas. Measurement of these gases for occupational health applications is typically been measured with a personal monitoring device.
Over the past twenty five years gas detection manufacturers have been steadily improving their measurement capabilities and reducing the size of the sensors to make the instrument as small as possible.
Have they reached the limits of their sensor technology to measure these gases accurately and with repeatability at such low concentrations. Currently some of these gas concentrations are at or below many manufacturers accurate measuring range.
If you cannot measure these gases in the workplace with personal monitoring devices then other methods will need to be used. Current technology may not allow this to occur without the risk of significant measurement errors. Of course, there is potential to develop new, more advanced sensor technology and create personal monitoring equipment that could allow us to decrease the TWA allowable limits but there are other considerations to consider.
How are we going to calibrate the instrument?
Today 0.1ppm CL2, 1ppm H2S, 0.9ppmHCN, 0.2ppmNO2 and 2ppmNO calibration gas are not available in small disposable cylinders most commonly used for occupational health/gas detection monitoring. All of these concentrations are too low for these type of cylinders.
Specialty gas mixtures in specially prepared high pressure cylinders will be required for these concentrations. Even then some of the concentrations may not be possible.
At these low concentrations the sensor capability will only be as good as the calibration gas used to calibrate the instrument. In some cases, more frequent calibration will be required, but if calibration gas is not possible what is the solution? Large industries may have the option of more expensive methods such as permeation tubes or gas generators but, unfortunately, that isn’t possible for many businesses.
The practical issues of measurement in the workplace should be considered when discussing reducing permissible levels. Not only is there a current lack of reliable technology to actually measure these new lower limits but there’s a risk that when or if it is available it will be out of reach to the consumer. If the purchase, maintenance and daily use of the gas monitoring equipment becomes more and more cost prohibitive for the user we may see businesses unable to comply with the new rules.
