Occupational Safety and Health question I need some understanding on...Willing to pay $

Briefly explain the following concepts related to exposure thresholds: time-weighted average, short-term exposure limit, exposure ceiling, and biological exposure indices.

I'm trying to write about 250  word answer. Don't have a background in this and I'm getting confussed how to start the answer. TLV could be long or short term exposure. Any help would be GREAT!

This is not a research or eassy==== but just a answer the question and after that I will add on so I can understand it and write the paper. Also I have another question about airborne contaminants that I will need to get answered.