2015 Highlights

Overall, air quality in Ontario has improved significantly over the past 10 years due to substantial decrease in harmful pollutants such as nitrogen dioxide, sulphur dioxide and carbon monoxide that are emitted by vehicles and industry.

There has also been a significant decrease in fine particulate matter which is emitted directly into the atmosphere as a by-product of fuel combustion or formed indirectly in the atmosphere through a series of complex chemical reactions. Fine particulate matter includes aerosols, smoke, fumes, dust, fly ash and pollen, and can have various negative health effects, especially on the respiratory system.

The continued decrease in these pollutants is due in part to Ontario’s air quality initiatives such as:

The following table shows the decreasing trend for airborne pollutant concentrations from 2006-2015 and emissions from 2006-2015:

Decreasing trend for provincial airborne pollutant concentrations from 2006-2015
Nitrogen dioxide 32%
Sulphur dioxide 48%
Carbon monoxide 53%
Fine particulate matter 25%
Decreasing trend for provincial emissions from 2006-2015
Nitrogen oxides 38%
Sulphur dioxide 44%
Carbon monoxide 32%
Fine particulate matter 11%

Ozone trends

Ozone is a pollutant formed when nitrogen oxides and volatile organic compounds (a group of pollutants from sources such as transportation, industrial activities, and use of solvents) react in the presence of sunlight. This means that formation and transport of ozone is very dependent on weather conditions and air pollutant concentrations.

Overall, annual ozone concentrations have increased three per cent over the past 10 years. Ozone concentrations can vary due to the season: winter ozone concentrations have increased by nine per cent while summer ozone concentrations have decreased by four per cent.

The winter increases are due mainly to rising global background concentrations of ozone. The summer increases are a result of reductions in emissions of pollutants such as nitrogen oxides and volatile organic compounds within Ontario and the U.S.

The following table shows a decreasing trend for six volatile organic compounds from 2005-2014, based on data collected at eight ambient air monitoring stations:

Decreasing trend for six volatile organic compounds from 2005-2014
1,3 Butadiene 62%
Benzene 42%
Toluene 64%
Ethylbenzene 79%
m-, p-xylene 85%
o-xylene 84%


This annual report, the 45th in a series, summarizes the state of ambient air quality in Ontario during 2015 and examines 10-year trends. It reports on the measured levels of six common air pollutants: ground-level ozone (O3), fine particulate matter (PM2.5), nitrogen dioxide (NO2), carbon monoxide (CO), sulphur dioxide (SO2) and total reduced sulphur (TRS) compounds, and how Ontario is performing compared to the province’s Ambient Air Quality Criteria (AAQC). This report also provides an overview and implementation of the new Air Quality Health Index (AQHI) and Air Quality Alert programs in Ontario, plus the monitoring of select volatile organic compounds (VOCs) in the province. Annual statistics, as well as 10-year trends of ambient air quality data are provided in the attached Appendix.

An AAQC is a desirable concentration of a contaminant in air, based on protection against adverse effects on health or the environment. The term “ambient” is used to reflect general air quality independent of location or source of a contaminant. AAQCs are most commonly used in environmental assessments, special studies using ambient air monitoring data, assessment of general air quality in a community and annual reporting on air quality across the province. AAQCs are set with different averaging times appropriate for the effect they are intended to protect against.
Contaminant1-hour AAQC8-hour AAQC24-hour AAQCAnnual AAQC
O380 ppbn/an/an/a
PM2.5n/an/a28 μg/m3(1)n/a
NO2200 ppbn/a100 ppbn/a
SO2250 ppbn/a100 ppb20 ppb
CO30 ppm13 ppmn/an/a

(1) Reference level based on Canadian Ambient Air Quality Standard (CAAQS).

Ontario continues to benefit from one of the most comprehensive air monitoring systems in North America, comprised of 39 monitoring sites across the province that undergo regularly scheduled maintenance and strict data quality assurance and quality control (QA/QC) procedures to ensure a high standard of data quality and data completeness. The data, which are collected continuously at these sites, are used to determine the current state of ambient air quality and are reported every hour on the Air Quality Ontario website.