There’s no single guideline for weld fume exposure. Instead, OSHA enforces Permissible Exposure Limits (PELs) for specific metals, reflecting limits over specific periods of time. For example, for aluminum, iron, and mild steel, the PEL is 5 milligrams of particulates per cubic meter of air (5 mg/m3) averaged over an 8-hour period. Metals that are more toxic, including chrome, manganese, stainless steel, nickel, and cadmium, have stricter PELs.
Many welding operations strive to stay within Threshold Limit Values (TLVs) spelled out by the American Conference of Governmental Industrial Hygienists (ACGIH). These estimates of the exposure level a worker can tolerate over a career, are based on current scientific research and may be lower than PELs. A reference to PELs and TLVs is the Pocket Guide to Chemical Hazards published by the National Institute for Occupational Safety and Health (NIOSH). Be aware, however, that your operation may have additional state and local requirements.