The airflow and capture velocity would depend on the configuration of the exhaust hood and the contaminant source location. The range of capture velocity may be in the range of 50 to 200 fpm. A good resource to investigate your application would be the Industrial Ventilation Handbook - by the American Conference of Governmental Industrial Hygienists. (Chapter 3 of the 1998 version)
They have a range for contaminants released with almost no velocity into quiet air (Evaporation from tanks, degreasing etc. 50 - 100 fpm. Disturbing air currents, highly toxic fumes, heavy use and locations with a small hood would see the higher end of the range utilized.
If the contaminant is released at low velocity into still air the range for capture increases to 100-200 fpm.