Airborne contaminants and exposure limits

Before anyone can choose a respirator, they need to know what is in the air, what physical form it takes and how much of it is acceptable. This lesson covers the forms airborne contaminants take, how particle size controls where they go in the body, and the exposure limits that define how much is too much in each country.

Particles: dusts, fumes, mists, fibers and bioaerosols

Particles are tiny solid or liquid pieces suspended in air. They are measured in micrometers (µm), one-thousandth of a millimeter; a human hair is roughly 50 to 100 µm across. The way a particle is formed largely determines its size, and size determines both where it lands in the lungs and how a filter captures it.

Occupational hygienists divide particles by how far they penetrate. The inhalable fraction is what can enter the nose and mouth at all. The thoracic fraction passes the larynx into the airways. The respirable fraction reaches the gas-exchange region, with a median cut-off of about 4 µm. Exposure limits often say which fraction they apply to, for example respirable crystalline silica or inhalable wood dust, and air sampling must collect the same fraction for the comparison to mean anything.

A practical rule follows: if you can see a dust cloud, there is almost certainly an invisible respirable fraction as well. Fumes and smoke are mostly invisible individually and appear only as a haze once concentrations are already high. The absence of visible dust does not mean the air is safe.

Gases and vapors

A gas is a substance that is gaseous at normal room temperature and pressure, such as carbon monoxide, chlorine, ammonia or nitrogen. A vapor is the gaseous form of a substance that is normally liquid or solid at room temperature, such as the vapor above a solvent, gasoline or mercury. Gases and vapors are individual molecules, far smaller than any particle, so particulate filters do nothing to stop them. They must be removed by chemical sorbents in cartridges or canisters, or the worker must be supplied with clean air.

Vapor concentration depends strongly on temperature. A warm solvent tank evaporates far faster than a cold one, and an enclosed space lets vapor accumulate. The maximum concentration a liquid can produce in still air is set by its vapor pressure. Acetone, with a high vapor pressure, can easily produce concentrations hundreds of times its exposure limit in a poorly ventilated room. That is one reason gas and vapor hazards can exceed the capacity of air-purifying respirators.

Some exposures combine forms. Spray painting produces both paint mist and solvent vapor, so a respirator needs a particulate prefilter and an organic vapor cartridge. Welding on galvanized or coated steel produces metal fume and may produce gases such as ozone, nitrogen oxides or carbon monoxide. Hot asphalt produces both fume and vapor. The hazard assessment must name every form present.

Concentration units

Gases and vapors are usually measured in parts per million (ppm) by volume. One ppm means one volume of contaminant in a million volumes of air. Particles, fumes and mists are measured in milligrams per cubic meter (mg/m³). Fibers are counted in fibers per cubic centimeter (f/cc). Exposure limits for gases are often given in both units, and you can convert between them at normal room conditions using the molecular weight.

Here \(MW\) is the molecular weight in grams per mole and 24.45 is the volume in liters occupied by one mole of gas at 25 °C and normal atmospheric pressure. The conversion matters when a limit is listed in one unit and a monitor reads in the other.

Types of exposure limits

Exposure limits are concentrations that most workers can be exposed to repeatedly without adverse health effects. They are not sharp lines between safe and unsafe, and they are not targets. Good practice is to keep exposures as far below the limit as reasonably practicable, especially for carcinogens and sensitizers. There are three main kinds of limit.

Some limits carry notations. A skin notation means the substance can be absorbed through the skin in significant amounts, so a respirator alone will not control total exposure; gloves and protective clothing may also be needed. A sensitizer notation warns that the substance can cause allergy. Carcinogen notations flag substances where exposure should be reduced to the lowest reasonably achievable level, not just below the number.

Who sets the limits

In the United States, the legally enforceable limits are OSHA's permissible exposure limits (PELs). Most PELs were adopted in 1971 from consensus values of the late 1960s and have never been updated; OSHA itself acknowledges that many are outdated and not protective. Other organizations publish more current recommended values that employers can and often should use.

Because PELs are often outdated, OSHA's own annotated Z-tables list Cal/OSHA PELs, NIOSH RELs and ACGIH TLVs side by side and encourage employers to consider the more protective values. A well-run program usually selects respirators against the most protective credible limit, not just the legal minimum. That choice also changes respirator selection, because the maximum use concentration depends on the limit used.

Mixtures

Workers are often exposed to several substances at once. When the substances affect the same organ in the same way, for example several solvents that depress the central nervous system, their effects are generally assumed to add. The combined exposure is assessed with the additive mixture formula.

Each \(C\) is the measured concentration and each \(L\) is the corresponding limit. If \(E_m\) is greater than 1, the mixture exceeds the limit even if each substance on its own is below its individual limit.

When substances act on different organs, their effects are assessed independently. Deciding whether effects are additive is a professional judgment and is one of the reasons complex mixtures should be assessed by an occupational hygienist.

Unusual work schedules

TWA limits assume roughly 8-hour shifts and 40-hour weeks, with 16 hours of recovery between shifts. Workers on 10- or 12-hour shifts receive a larger daily dose and have less recovery time. For substances with chronic effects, hygienists reduce the limit for long shifts.

The simplest and most widely cited method is the Brief and Scala model, which reduces the limit by a factor that accounts for both longer exposure and shorter recovery.

Here \(h\) is the length of the shift in hours. The adjusted limit is the normal limit multiplied by \(RF\). Brief and Scala is deliberately conservative and is not applied to ceiling limits or to substances that act acutely.

OSHA's PELs do not contain a formal adjustment for unusual shifts, but OSHA guidance and industrial hygiene practice commonly use Brief and Scala or toxicokinetic models when assessing exposure on extended shifts. Some substance-specific standards define exposure as an 8-hour TWA regardless of shift length, so long shifts must be sampled and evaluated carefully.