RoboVent Results
Home / Blog / Understanding Exposure Limits: PELs, TLVs, SECALs and More

Understanding Exposure Limits: PELs, TLVs, SECALs and More

June 19, 2018

RoboVent_AirQuality_Regulations_1920xBy Trevor Kendall, VP Distribution Sales

Indoor air quality (IAQ) standards can include some confusing language and acronyms. What's the difference between a PEL, an REL and a TLV? And which ones do you need to follow? Here's a handy guide to help you sort it all out.


Who Sets the Standards for Air Quality?

In the U.S., the Occupational Health and Safety Administration (OSHA) sets exposure limits and air quality standards for manufacturing environments. Most other countries have similar IAQ standards set by their equivalent governing body. Air quality standards set by OSHA and other similar governing bodies are legally binding; failure to follow them can result in large fines and penalties for manufacturers.

Other organizations, such as the National Institute of Occupational Safety and Health (NIOSH) and the American Conference of Government Industrial Hygienists (ACGIH), have their own air quality guidelines. Unlike OSHA standards, these are not legally binding.

NIOSH's recommended exposure limits (RELs) are considered to be authoritative federal guidelines that are used by OSHA and other government agencies to set federal standards and regulations. NIOSH sets its guidelines based solely on evidence of human harm. They are often lower—sometimes much lower—than the legally mandated exposure limits. That is because OSHA also takes technical feasibility and economic considerations for employers into account when setting the legal standard.

ACGIH also sets their recommendations based solely on human health impact. Their guidelines often call for lower limits than those set by OSHA. While not legally enforceable, ACGIH recommendations are often considered to be best practices to strive for both in the U.S. and internationally.

Air Quality Regulations and You

There are a few different regulations that U.S. manufacturers need to be aware of when it comes to air quality.

  • The OSHA General Duty Clause says that employers "shall furnish to each of their employees a place of employment which is free from recognized hazards that are causing or are likely to cause death or serious physical harm." Air quality falls under this clause; employers must make sure that air is comfortable to breathe and does not create a health or safety risk for workers.
  • OSHA CFR 1910 Subpart Z details permissible exposure limits (PELs) for most common airborne contaminants such as hexavalent chromium, cadmium, aluminum, or beryllium. These are generally given as either a ceiling (meaning concentrations cannot exceed the ceiling level at any time) or as a Time Weighted Average (TWA) calculated over an 8-hour shift. PELs are set for each potentially hazardous substance individually; manufacturers can find PELs for specific substances under Chemical Sampling Information on the Department of Labor website. Dusts that do not have their own PEL must still be controlled under the "Total Dust/Particulates Not Otherwise Regulated" standard.
  • Because many airborne dusts are combustible, manufacturers should also be aware of OSHA's regulations for combustible dusts. It's important to ensure that dust is not able to accumulate in any enclosed area, which can lead to an explosion.

OSHA sets different standards for different types of industries. These are generally categorized as Shipyard, Construction, or General Industry. Most manufacturers and fabrication shops will need to follow the General Industry exposure limits.

Manufacturers should also be aware of any local air quality or industrial hygiene standards that may govern their actions in a particular state or locality. For example, California has set exposure limits for many substances that are lower than the federal OSHA PELs. When there are conflicting national and local air quality standards, manufacturers must follow the more stringent limit.

Sorting Out Air Quality Alphabet Soup

All of these different organizations have created a virtual alphabet soup of acronyms when it comes to air quality. Here are the ones manufacturers are most likely to be faced with.

  • AL: Action Level. This is the exposure level that is considered to create a hazard to workers. If the AL is exceeded, employers must conduct exposure monitoring, initiate a medical surveillance program for employees who are at or above the AL on 30 or more days per year, and provide a respirator to any employee that requests one. Not all substances have an AL; ALs are generally used for substances with a high risk of harm or where PELs may not provide adequate protection due to the technical difficulty of controlling the substance. Action limits are legally enforceable.
  • PEL: Permissible Exposure Limit. OSHA sets PELs for toxic substances that workers may be exposed to in an occupational setting. Manufacturers must keep exposures at or under the PEL for each substance workers are exposed to. Most PELs are measured as a time-weighted average concentration that must not be exceeded during any 8-hour work shift of a 40-hour workweek. Some PELs are set as ceilings, meaning concentrations cannot exceed this level at any time. PELs are legally enforceable.
  • REL: Recommended Exposure Limit. NIOSH sets RELs for substances based on available scientific literature on their biological effects on different body systems. RELs may be set as a time-weighted average or as a ceiling. They are used as guidelines by OSHA and other government bodies in setting legal standards, but are not legally enforceable themselves.
  • SECAL: Some manufacturers using specialized processes may have different exposure limits known as Separate Engineering Control Air Limits. These limits are often higher than the standard PEL due to technical difficulty in meeting the PEL in some environments. These exposure limits are legally enforceable.
  • STEL: Short Term Exposure Limit. This is an average exposure limit over a short time, usually 15 minutes. STELs provide an upper limit in concentrations that should not be exceeded at any time, even if the TWA across the entire shift is still within the PEL. STELs set by OSHA are legally enforceable; NIOSH STELs are not.
  • TLV: ACGIH sets Threshold Limit Values (TLVs®) for exposure to toxic airborne substances. The TLVs are set based on available scientific literature into health effects, which ACGIH codifies as Biological Exposure Indices (BEIs®). ACGIH TLVs are recommendations and are not legally enforceable.
  • TWA: Time-Weighted Average. This is a method of calculating exposure levels that looks at the average concentration of the substance in the air over a standard 8-hour shift.

Air Quality Standards and Guidelines for Selected Elements and Compounds


(General Industry)

(General Industry)
Beryllium and Beryllium Compounds N/A 0.002 mg/m3 TWA 
0.005 mg/m3 Ceiling 
0.025 mg/m3 Peak (30 minutes)
0.0005 mg/m3 Ceiling  0.00005 mg/m3 TWA (inhalable particulate matter)
Cadmium 2.5 μg/m3 Action Level 5 μg/m3 TWA  Not established

0.01 mg/m3 TWA 

0.002 mg/m3 (respirable particulate fraction) 

Hexavalent Chromium 2.5 μg/m3 TWA  5.0 μg/m3 TWA 0.0002 mg/m3 (8 hr TWA) 

0.05 mg/m3 (water-soluble Cr(VI) compounds, as Cr) TWA;

0.01 mg/m3(insoluble Cr(VI) compounds, as Cr) TWA

Manganese Fume N/A 5 mg/m3 Ceiling 1 mg/m3 TWA  0.2 mg/m3 (respirable particulate matter) TWA
Nickel (Metal and Insoluble Compounds) N/A 1 mg/m3 TWA 0.015 mg/m3 TWA 1.5 mg/m3 (inhalable fraction) TWA