Exposure to a substance is uptake into the body. The exposure routes are:
Many thousands of substances are used at work but only about 500 substances have Workplace Exposure Limits (WELs) listed in EH40 workplace exposure limits
You can only do this by monitoring. This means measuring the substance in the air that the worker breathes while the task is underway. A guidance sheet, Exposure measurement: Air sampling G409 tells you what to expect from a competent consultant who provides monitoring services.
You can check whether you are using the right control measures. If you are, exposures are likely to be below the WEL. See COSHH essentials