In Alberta, the hourly pollutant concentrations are also compared against Alberta’s Ambient Air Quality Objectives (AAQOs). If the following pollutant thresholds are exceeded, the federal AQHI value is overridden with a High or Very High risk value (7 or greater):

• 80 micrograms per cubic metre for fine particulate matter (PM2.5)
• 172 parts per billion for sulphur dioxide
• 159 parts per billion for nitrogen dioxide
•  82 parts per billion for ozone
• 13 parts per million for carbon monoxide
• 1 part per million for hydrogen sulphide and total reduced sulphur  

Children, the elderly and those with compromised breathing can be particularly sensitive to elevated risks.

Below is some background on the more common air borne pollutants of concern (adapted from https://floridadep.gov/air/air/content/air-quality-101). The Alberta Ambient Air Quality Objectives list many more.

Particulate Matter - PM2.5, PM10

Particulate pollution is the general term used for a mixture of solid particles and liquid droplets found in the air. This pollution is made up of a number of components, including dust particles, acids (such as sulphates and nitrates), organic chemicals, metals, smoke, haze, and allergens (such as fragments of pollen or mould spores).

The size of the particle is directly linked to their potential for causing health problems. Small particles pose the greatest threat as they penetrate deep into the tissue of our lungs and are hard to expel. PM2.5 describes the smallest particles of concern; they are "fine particles" (such as those found in smoke and haze), which are 2.5 micrometers in diameter or less. "Coarse" particles, or PM10, describe particles greater than 2.5, but less than or equal to 10 micrometers in diameter.  Ten micrometers are about one-seventh the diameter of human hair.

Particulate pollution originates from many different stationary and mobile sources as well as from natural sources. PM2.5 typically result from emissions of fuel combustion from motor vehicles, power generation and industrial facilities, as well as from wildfires and residential fireplaces.  In other cases, gases such as sulphur dioxide, nitrogen oxides and volatile organic compounds interact with other compounds in the air to form PM2.5.  Coarser particles (PM10) are generally emitted from sources such as vehicles traveling on gravel roads, materials handling, crushing and grinding operations, and windblown dust.

Sulphur Dioxide

Sulphur dioxide (SO₂) is produced by power plants and industries that burn fossil fuels containing sulphur, such as coal and oil, high-sulphur diesel and by the phosphate industry through its production of sulphuric acid.  SO₂ is may also be present in Alberta where sour gas containing H₂S is flared or incinerated.  SO₂ is irritating to the lungs and can result in a higher incidence of respiratory disease. 

Nitrogen Dioxide (NO₂)

Nitrogen dioxide, which is a primary component of nitrogen oxides (NOx), is produced when fuel is burned in motor vehicles, trains, power plants, industrial boilers and other sources.  Diesel engines produce more NOx relative to gasoline engines and this is one of the reasons for the phase out of diesel vehicles in Europe.

Nitrogen dioxide can place a strain on the heart and respiratory system and can increase a person's susceptibility to respiratory infections.

Ozone (Smog)

Ozone occurs naturally near the earth's surface. However, man-made emissions of VOCs (volatile organic compounds) and NOx can cause additional ozone, the primary component of urban smog. This additional ozone, which can more than triple the amount of natural ground-level ozone, can cause health and environmental damage. Ozone builds up near the ground through a series of complex chemical reactions involving VOCs and NOx in the presence of sunlight. VOCs are produced by fuel combustion in engines and industrial operations, chemical manufacturing operations, evaporation of solvents in consumer and commercial products and evaporation of volatile components from gasoline and other fuels.

Ozone concentrations can reach unhealthy levels when the weather is hot and sunny with relatively light winds. Even at relatively low levels, ozone may cause inflammation and irritation of the respiratory tract, particularly during physical activity. The resulting symptoms can include breathing difficulty, coughing and throat irritation. Breathing ozone can affect lung function and worsen asthma attacks.

Carbon Monoxide (CO)

Carbon monoxide is produced primarily by motor vehicles. It can reduce a person's ability to think clearly and causes visual impairment and headaches if high enough concentrations are experienced for a long period of time.

Mercury

Mercury is a naturally occurring element present throughout the environment. It becomes a toxic air pollutant when released into air, water and soil by human activity.  Reactive, inorganic mercury is emitted to the atmosphere primarily from coal-burning facilities and incinerators that combust mercury-containing wastes.  Air currents and rainfall convey this mercury from the atmosphere to the earth's surface. Some of the air borne mercury ends up in wetlands, lakes and streams where bacteria convert a portion of it into methylmercury, a toxic form that builds up (bioaccumulates) in the tissues of animals at each link in the food chain.

Lead (Pb)

For many years, lead was primarily emitted to the atmosphere from motor vehicles that burned leaded gasoline. Lead can affect the central nervous system and lead to anemia. The young and the elderly are most susceptible to the harmful effects of lead.  Fortunately lead has been phased out of gasoline in most places, but it is still present in some developing countries.