## Units of Measurement Used in Industrial Hygiene-September 2022

When reading industrial hygiene reports there is often confusion about the units used for results. Here are some terms used in air and surface sampling, noise,  and ventilation that may be helpful.

• Milligrams per cubic meter (mg/m3) is the weight of a particle, fume, or mist in a volume of air. Often used to express the OSHA Permissible Exposure Limit, (PEL) American Conference of Industrial Threshold Limit Value, National Institute of Occupational Safety & Heath Recommended Exposure Limit, or other occupational exposure standards.
• Micrograms per cubic meter (ug/m3) are similar to mg/m3 but used for smaller quantities of contaminants. 1 mg = 1000 ug.
• Parts per million (ppm)  is a weight to weight ratio used to describe concentrations of gasses in air. Units of mass of a contaminant per million units of total mass. It is also used for occupational exposure limits  but only for gasses.
• X ppm = (Y mg/m3)(24.45)/(molecular weight) at 25 degrees C.
• Liters per minute (lpm) flow rate of an air sampling pump.
• Micrograms per square feet (ug/ft2) is the weight per surface area unit used in HUD/EPA standard for lead on window sills in housing projects. It is also used as a standard for lead wipe samples. Other types of surface sampling are expressed in micrograms per cubic centimeter (ug/cm2).
• Decibel (dB) is a logarithmic unit used to measure sound pressure or the sound level. A-weighted scale (dBA) is  an expression of the relative loudness of sounds in air as perceived by our ears and is used by OSHA for the PEL of 90 dBA and Action Level of 85 dBA.
• Noise-Dose is the percentage of allowable exposure incurred during the monitoring period. It is a linear scale as opposed to a logarithmic scale for decibels. For example, the OSHA Action Level  is 85 dBA for 8 hours of exposure is equivalent to a 50% dose.  The OSHA PEL of 90 dBA for 8 hours of exposure equals a 100% dose.
• Feet per minute (fpm) is used to measure the air velocity at the opening of a duct, paint spray booth, or otherwise in ventilation studies.
• Cubic feet (ft3) is the volume of air in a duct.
• Cubic feet per minute (cfm) is the volume of air passing through a duct per time.
• Square feet (ft2) is the area of a duct.
• Inches of water is used to measure static or velocity pressure inside a duct or HVAC system.

There are many other units of measure used in industrial hygiene. These may include ones for ionizing and non-ionizing radiation, vibration, heat and cold, ergonomics, etc. It is important that safety professionals are familiar with all units and terms that may appear in an industrial hygiene survey to adequately understand and implement the findings and controls needed.

For more information on this topic and to discuss your company’s safety and industrial hygiene needs call OccuSafe at (214) 662-6005 or visit us at www.occusafeinc.com.This newsletter is published monthly by OccuSafe Industrial Hygiene & Safety Services, Inc. Feel free to forward it to friends and colleagues or see past newsletters at occusafeinc.com/category/newsletter/

OccuSafe Industrial Hygiene & Safety, Inc. provides skills and expertise to recognize, evaluate and control hazards and injuries in the areas of industrial hygiene, occupational safety and health. OccuSafe services companies of all sizes in a range of industries.