Certain chemicals in use in the laboratory, such as formaldehyde, are hazardous if your exposure to them is prolonged.
The amount of the chemical to which you can be exposed before possible danger is called the threshold limit value.
Monitoring badges are used from time to time to measure your exposure. These are worn in the "breathing zone" for a certain period of time--often eight hours (for long-term exposure) or fifteen minutes (for short-term exposure).
Based on the results of this monitoring, additional personal safety measures, such as ventilation or face-fitted masks, may be implemented for your protection.