How are lead dust clearance test results expressed?

• A. Parts per million (ppm)
• B. Milligrams per square meter (mg/m²)
• C. Micrograms per square foot (µg/ft²)
• D. Grains per gallon

Answer: (C)

Lead dust clearance results are reported as the amount of lead per unit area on a surface, which is naturally expressed in micrograms per square foot. This makes sense because wipe samples are taken from a defined, measured area, and the lab measures how many micrograms of lead are present within that specific area. Expressing the result as µg/ft² allows direct comparison to the clearance criteria, which are set per square foot of surface.

Other units don’t fit this scenario. Parts per million is a concentration within a solution or bulk material, not a surface area measurement. Milligrams per square meter is a metric-area unit, but the standard in lead dust clearance in many contexts is micrograms per square foot. Grains per gallon is used for liquids, not surface dust.