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, fibres and bioaerosols

Particles are tiny solid or liquid pieces suspended in air. They are measured in micrometres (µm), one-thousandth of a millimetre; 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 vapours

A gas is a substance that is gaseous at normal room temperature and pressure, such as carbon monoxide, chlorine, ammonia or nitrogen. A vapour is the gaseous form of a substance that is normally liquid or solid at room temperature, such as the vapour above a solvent, gasoline or mercury. Gases and vapours 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.

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

Some exposures combine forms. Spray painting produces both paint mist and solvent vapour, so a respirator needs a particulate prefilter and an organic vapour 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 vapour. The hazard assessment must name every form present.

Concentration units

Gases and vapours 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 metre (mg/m³). Fibres are counted in fibres per cubic centimetre (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 litres 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 Canada, exposure limits are set by each province and territory and by the federal government for federally regulated workplaces. Most jurisdictions base their limits on the threshold limit values (TLVs) published by the American Conference of Governmental Industrial Hygienists (ACGIH), a non-governmental scientific body, but they adopt them in different ways and on different schedules.

Because limits differ, the same exposure can be compliant in one province and not in another. A company working across provinces needs to use the limit in force where the work happens, and many choose to apply the lowest of the applicable values everywhere.

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.

Ontario's ministry publishes an adjustment tool for irregular work shifts based on the Quebec model developed by the IRSST, which classifies substances by their health effects and applies different adjustments accordingly. Other provinces provide guidance or require the employer to consider extended shifts in their exposure control plan. Check the method your jurisdiction expects.