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Concentration Units

Last updated: September 26, 2018

What Does Concentration Units Mean?

Concentration units are units that express the relative concentration of a given substance within a mixture. There are a variety of different concentration units, some of which express the same information in different ways, and some of which provide different information about the relative amount of chemical that is within a mixture.

The four main categories of concentration units are mass concentration, molar concentration, number concentration, and volume concentration.

Safeopedia Explains Concentration Units

Concentration units are used in occupational safety to express the exposure limits for specific hazardous substances and to define the amount of toxic substance within a given mixture. The occupational health and safety standards that govern exposure limits are provided in specific concentration units for each substance so that exposure can be understood so long as measurements are converted into the unit provided. Hazard communication and labeling requirements, such as the preparation of safety data sheets, must include information about the concentration of hazardous chemicals unless that information reveals proprietary information about the chemical.

OSHA expresses its occupational exposure limits (OELs) for chemical substances in mass concentration units, which express the mass of a constituent substance in a mixture divided by the total volume of the mixture. OSHA OELs, which are called permissible exposure limits (PELs), are listed with the unit milligrams per meter cubed (mg/m3). The Center for Disease Control’s National Institute for Occupational Safety and Health (NIOSH) uses the same unit for its recommended exposure limits (RELs).

OSHA’s use of a mass concentration unit to set limits for the amount of substance (mg) allowed within a particular amount of space (m3) means that exposure limits are set as a fixed content that does not have to be corrected for temperature or pressure. This is because the relationship between temperature, pressure, and volume is fixed. In contrast, when the amount of a chemical substance is provided in numerical units such as parts per million (PPM), the temperature and pressure of a given environment must be taken into consideration.

According to NIOSH, numerical units are typically given with a reference to an ambient environmental temperature of 25 degrees Celsius (°C) and a pressure of 760 millimeters of mercury (mm Hg). In work spaces that don’t use this temperature and pressure, the volume concentration of particles within the space would be different, and so the number of particles within the space would be, too.

Safety data sheets and other materials may use different concentration units than OSHA. For instance, a biomedical company that makes an arsenic acid might list the concentration of arsenic in the acid using a mass unit measurement of 35 µg As/L (micrograms of arsenic per liter). The other common concentration units used in chemical labeling—molarity and volume concentration—are expressed in terms of moles of substance per volume and percent of substance per volume.


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