Industrial ventilation is an example of a basic but effective engineering control that is used to avoid workers' exposure to toxic or hazardous materials. Many substances are hazardous when their dusts, vapors, fumes, or gases enter the air. When airborne contaminant levels become dangerous, respirators are used, but the first line of defense against such hazards is ventilation. Federal regulation requirements can be found at 29 CFR 1910.94. OSHA expects companies to use engineering controls like ventilation as the first line of protection for workers.