What unit is used to express exposure limits?

• A. ppb
• B. percent
• C. ppm
• D. mg/m3

Answer: (C)

Concentration of a gas in air is most often expressed as parts per million. This unit shows how many molecules of the contaminant are present for every million molecules of air, which makes it a practical and comparable way to set exposure limits across different gases. Regulatory agencies commonly publish exposure limits in ppm for gases and vapors, because it directly reflects the ratio in the breathing zone and stays convenient across varying temperatures and pressures.

Other units don’t fit the typical way these limits are communicated as well. Parts per billion is used only for extremely trace levels, while percent by volume is much less practical for standard occupational limits. mg/m3 is a mass-per-volume measure and is more common for particulates or when a mass-based dose relation is needed, rather than for gases in air.