The Odour Index is calculated separately from the Air Quality Index, and is used to indicate the presence of pollutants that, aside from their odour, do not, at low concentrations, pose a concern to health or the environment. Pollutants that create an odour problem are generally comprised of compounds that contain sulphur in its reduced chemical state.

An Odour Index of 26 or greater indicates that reduced sulphur compounds are present in a total concentration greater than the "Desirable" Provincial Ambient Level. An index of 51 or greater indicates concentrations in excess of the "Interim" Provincial Ambient Level.

The Odour Index for a given Reduced Sulphur concentration is determined from a set of linear interpolations starting at zero and having break-points at values of 25, 50 and 100. The ambient values of the break-point concentrations measured over 1-hour and 24-hour averaging periods are as follows:

When we talk about the diameter of PM, we use microns (micrometres) — "µm" (so PM_2.5 and PM₁₀).

When we refer to the concentration of PM, we use micrograms — "µg" ("µg/m" is the concentration term we use).

Particulates, also known as particulate matter or PM, are very small particles that come from a variety of natural and human-produced sources — and a range of sizes. Two of the most common sizes of classifications are PM₁₀ and PM_2.5.

PM₁₀ represents particulate matter less than 10 microns in diameter, while PM_2.5 (a subset of PM₁₀) represents those particles of diameter less than 2.5 microns. Often PM_2.5 is referred to as the fine fraction of PM₁₀. The fraction of PM₁₀ containing particles with diameters greater than 2.5 mcirons is termed the coarse fraction. (Microns are also referred to as "micrometres".

The smaller particulates — PM_2.5 — most frequently are the result of combustion activities and chemical reactions in the atmosphere. When emitted directly from their source, these particles are considered "primary". The particles that are formed as a result of chemical reactions in the atmosphere are considered "secondary".

The larger, more coarse particulates generally come from natural sources, such as wind-blown dust, sea salt spray and vegetation. They are also emitted from human-produced sources, including road dust, and various grinding activities. Because of their relatively large size, these particles tend to settle out of the atmosphere much more quickly than fine particulates, in a few hours to a few days. Fine particulates, on the other hand, can stay in the air for days to weeks.

In our bodies, too, coarse particulates are not a major problem, they tend to be removed in the upper portions of our respiratory system — our nose and throat — before they can get into our lungs. Fine particulates, however, can penetrate deep into our lungs, collecting in the tiny air sacs (called "alveoli") where oxygen enters the bloodstream. As a result, they can cause breathing difficulties and sometimes permanent lung damage.

Fine particulates are much more complex chemically than coarse particulates, a factor which may affect their inherent toxicity. They are often composed of harmful substances — for example, toxic trace metals like lead and cadmium, polycyclic aromatic hydrocarbons (PAHs: it is suspected that some PAHs cause cancer), volatile organic compounds (VOCs), carbon monoxide and formaldehyde.