Answer:
the time and concentration that you are exposed to.
Explanation:
In some jobs, when you are in contact with some hazardous substances like acid vapors or rock dust, there most be a federal law that explain the time and concentration as a maximum in your job.
For example, if you work with some radioactive material or machine, like for example X rays, you can not work more than certain hours because if you do double turn or take more hours that you need to be, radiation will be accumulated in your bones until you can have cancer. Something similar occurs with other chemical substances.
Hope this info is useful for you.