Why time-weighted averages are used

A TWA combines concentration and duration over a stated reference period. For GB WELs the long-term reference period is eight hours, which need not equal the actual shift length. Calculate exposure using the applicable method, with competent advice for unusual or extended patterns.

A PEL is a US regulatory term, not a synonym for every national OEL and not a guarantee of no adverse effects. Where a short-term limit also exists, both periods must be assessed. A low long-term average must not be used to justify uncontrolled peaks.

Do not automatically apply a 40-hour-week formula or invent a shift adjustment. The relevant national system, substance information, exposure pattern and susceptibility determine the assessment.

Limitations of exposure limits

• Exposure limits relate to personal monitoring.
• Most WELs cover inhalation only. They do not address absorption after swallowing or through the skin or eyes. For some chemicals the main route is absorption into or through the skin, and in those cases biological limit values are used in place of, or as well as, inhalation limits.
• WELs do not take account of individual susceptibility.
• A limit for one substance does not by itself resolve mixed exposure. EH40 describes combined-effect assessment, including an additive approach where appropriate, with important exclusions. Use a competent evaluation of all constituents and interactions rather than assume either that mixtures are covered automatically or can never be assessed.
• An organisation may believe limits are being met while employees work with controls that no longer work.
• Some limits are only guidelines.
• Conditions that add stress to the body, such as ultraviolet radiation, high temperature, pressure and humidity, may increase the toxic response to a substance. Specialist advice may be needed to evaluate these effects.
• Limits reflect the evidence and regulatory process used to establish them and may change. Check the current official list and assess residual risks; do not assume every revision lowers a value or that every existing limit is automatically adequate for every individual.

Comparing measurements with recognised standards

Workplace exposure limits are meant to protect workers from excessive exposure to toxic chemicals. They typically include safety margins so that workers are not overexposed, and employers generally must make sure the limits are not exceeded. The most common limits are:

Summary notes for Element 7 · Cheat sheet · Glossary. A study aid, not published or endorsed by NEBOSH.