Toxicity units of measurement

The toxicity of an inhaled gas is commonly indicated as a:
Threshold Limit Value (TLV) expressed in parts per million (ppm) of gas by volume in air.

TLV concentrations are those believed to be safe for an 8 hour exposure day for an indefinite period.

TLV's should not be regarded as fine lines between safe and dangerous concentrations, nevertheless, they should be used on board ship as values 'below' which it is safe for continuous exposure and 'above' which it 'may be unsafe' even for short exposure. If the gas or vapor concentration is greater than the TLV, breathing apparatus should be worn if entry into a compartment containing such a concentration is necessary.