Some gases are toxic and can cause injury or death if inhaled, absorbed through the skin or swallowed. The degree of toxicity varies from gas to gas. For example, carbon monoxide is a toxic gas emitted by automobile exhaust systems. It can be harmful to individuals exposed to this gas in concentrations of more than 50 parts per million (ppm) over an 8 hour period. Arsine, on the other hand, is a highly toxic gas. It can be harmful to individuals exposed to this gas in concentrations of more than 0.05 ppm over an 8 hour period. Concentrations of 50 ppm for carbon monoxide and 0.05 ppm for arsine are measures of toxicity called Threshold Limit Values (TLVs) or Permissible Exposure Limits (PELs) for these gases. [NOTE: TLVs are measures of toxicity established by the ACGIH while PELs are measures of toxicity established by OSHA. Because the OSHA PEL for a given substance may or may not be the same as the TLV established by ACGIH].