I believe OSHA publishes maximum concentrations for worker exposure for a wide range of substances- try some research there.
If the product in question is flammable, the concentration should be kept well below the lower explosive limit (assuming any personel are using supplied air!).
It help to tell more about your particular application, and where you're at.
The application is ventilation from dust made from operators weighing up raw materials in a specified room. Most of the materials aren't flammable, but dusty.
You might for starters check the MSDS for the products in question. If no help there, perhaps actually ask the manufacturers if there are any safety concerns with inhalation. Probably unlikely, but you hear of grain dust explosions, so make sure that can't happen.
Otherwise, sounds like you're into the Industrial Hygiene field- look for someone with more expertise there.
Some products you wouldn't think about can be bad news. Common beach sand dusted into the air gives you silicosis, about like black lung disease- and what could be more harmless than sand?
There are recomendations in Osha, 2003 international fire code, and 2003 international mechanical code. There are also recomendations in NFPA 68 and NFPA 69. They are almost all identical so if you can get a hold of a couple of references you should be good.
If I was really good I could tell you the sections but I am no code expert.
Here's a publication by the American Conference of Governmental Industrial Hygienists that I've used and seen referenced quite often. It covers all aspects of ventilation system design, types of hoods and gives recommendations for capture velocities.
Industrial Ventilation: A Manual of Recommended Practice
