What are Control Limits?
Control Limits are the maximum number of asbestos fibres that an employee can be exposed to during their work without using specialised breathing aparatus (RPE). They are calculated using different periods of exposure to ensure that neither short or long-term exposure becomes hazardous.
Employers are required, by law, to prevent employees from exposure that exceeds the Control Limits. Normally this applies to asbestos workers (such as surveyors and asbestos strippers) but it can also be used in other working situations where operatives are known to work in close proximity to asbestos containing materials (ACM) i.e plumbers, electricians and roofers. They would also be used in a situation where ACM is known to be present in the normal working environment i.e Asbestos ceiling tiles.
How is exposure measured?
Asbestos exposure is measured by testing the air in the working area. Air is drawn through a filter in a specialised 'air pump' and then an asbestos analyst counts the number of fibres present. The analyst can thus calculate the concentration of asbestos fibres in the air.
So if, for example, the pump collects air for 10 minutes, sucking through 20 mililitres (ml) of air in that period and the analyst finds 200 fibres (f) the concentration is calculated as 10 fibres per mililitre in 10 minutes (total fibres/total mililitres expressed as 20f/ml).
Table 1. Asbestos exposure Control Limits
| Asbestos Type |
4 hour limit |
10 minute limit |
12 week |
| Chrysotile ONLY |
0.3 f/ml |
0.9 f/ml |
72 fibre hrs/ml2* |
| Any other |
0.2 f/ml |
0.6 f/ml |
48 fibre hrs/ml2* |
*Calculated by adding together all exposures over 12 weeks
How do I keep within these limits?
Clearly those at highest risk work within the asbestos management industry. Such operatives are given adequate training to ensure that they know when they will be exposed and what they need to do to avoid exposure. In most instances their employers will also be licensed or accredited and will need to demonstrate that they regularly carry out air testing and keep precise records of exposure.
Any other workers that are likely to be exposed to asbestos either through the presence of known ACM in their workplace or, through work that is intrusive by nature (i.e building and demolition) should be given adequate training to ensure that they recognise asbestos materials and avoid exposure. They should also know how to decontaminate themselves should exposure occur. If high-risk ACMs are known to be present in working areas then it is worth getting air tests carried out to ensure that any fibre release is within limits.