Toxic Gas Measurement – Warning, Danger, STEL, and TWA alarms IQ Force™ Gas Detector Reference Manual Appendix Appendix A Toxic Gas Measurement – Warning, Danger, STEL, and TWA alarms Many toxic substances are commonly encountered in industry. The presence of toxic substances maybe due to materials being stored or used, the work being performed, or maybe generated by natural processes. Exposure to toxic substances can produce disease, bodily injury, or death in unprotected workers. It is important to determine the amounts of any toxic materials potentially present in the workplace. The amounts of toxic materials potentially present will affect the procedures and personal protective equipment that must be used. The safest course of action is to eliminate or permanently control hazards through engineering, workplace controls, ventilation, or other safety procedures. Unprotected workers may not be exposed to levels of toxic contaminants that exceed Permissible Exposure Limit (PEL) concentrations. Ongoing monitoring is necessary to insure that exposure levels have not changed in away that requires the use of different or more rigorous procedures or equipment. Airborne toxic substances are typically classified on the basis of their ability to produce physiological effects on exposed workers. Toxic substances tend to produce symptoms in two time frames. Higher levels of exposure tend to produce immediate (acute) effects, while lower levels of long-term (chronic) exposure may not produce physiological symptoms for years. Hydrogen sulfide (HS) is a good example of an acutely toxic substance which is immediately lethal at relatively low concentrations. Exposure to a 1,000 ppm (parts per million) concentration of HS in air produces rapid paralysis of the respiratory system, cardiac arrest, and death within minutes. Carbon monoxide (CO) is a good example of a chronically toxic gas. Carbon monoxide bonds to the hemoglobin molecules in red blood cells. Red blood cells contaminated with CO are unable to transport oxygen. Although very high concentrations of carbon monoxide maybe acutely toxic, and lead to immediate respiratory arrest or death, it is the long term physiological effects due to chronic exposure at lower levels that take the greatest toll on affected workers. This is the situation with regards to smokers, parking garage attendants, or others who are chronically exposed to carbon monoxide in the workplace. Exposure levels are too low to produce immediate symptoms, but small repeated doses reduce the oxygen carrying capacity of the blood overtime to dangerously low levels. This partial impairment of the blood supply may lead overtime to serious physiological consequences. Because prudent monitoring programs must take both time frames into account, there are two independent exposure measurements and alarm types built into the IQ Force design.