If you've followed correct procedures when working with industrial chemicals, you've read the MSDS before use. Something you may have noticed is "TLV." What exactly does this mean?
TLV is an abbreviation of threshold limit value. The threshold in question is the limit of how much of the toxin you can safely be exposed to. Typically, you'll see "ppm," meaning parts per million, after the number. But you might see "mcm," meaning milligrams per cubic meter.
Along with the exposure amount, you should see an exposure time (e.g., 8 hours) because TLV is a time weighted average. Plan your job around this TLV so you don't exceed the allotted exposure time.
The higher the TLV, the safer the chemical is. But a high TLV doesn't mean a chemical is harmless. Use the TLV to limit your exposure, not to limit your adherence to safe practices. Note also that contamination of your skin or clothing usually extends the effective exposure time.
