Ozone Depletion, Ground-Level Ozone and Smog

Most of Earth's ozone sits in the stratosphere, mainly between about 15 and 35 km up. There, high-energy ultraviolet (UV) light splits oxygen molecules into single oxygen atoms, which then join other O₂ molecules to form ozone. Ozone in turn absorbs UV and is split apart, then re-forms, in a natural cycle that turns harmful UV into heat. The ozone layer blocks all of the most dangerous UV-C and most UV-B. UV-B causes sunburn, skin cancers and cataracts, and damages crops and ocean plankton.

Chlorofluorocarbons (CFCs) are halocarbons invented in the 1920s and 1930s and widely used in refrigerators, air conditioners, spray cans and foam. They are unreactive and non-toxic near the ground, which seemed ideal, but that stability let them drift up into the stratosphere over several years. There, strong UV breaks chlorine atoms off the CFC molecules. A chlorine atom reacts with ozone, and the product reacts again in a way that sets the chlorine atom free to attack another ozone molecule. Because the chlorine is not used up (it acts as a catalyst), one chlorine atom can destroy many thousands of ozone molecules before it is removed. Bromine from halons (used in fire extinguishers) works the same way and is even more destructive.

In 1974 Mario Molina and F. Sherwood Rowland warned that CFCs could damage the ozone layer (they shared the 1995 Nobel Prize in Chemistry with Paul Crutzen). In 1985, British Antarctic Survey scientists reported a dramatic thinning over Antarctica every southern spring: the ozone hole. It forms there because the extremely cold Antarctic winter produces ice clouds in the stratosphere on which chlorine is converted into forms that destroy ozone rapidly once sunlight returns. Thinning also occurs over the Arctic in some cold years, which matters for northern Canada.

Ground-level ozone is not released directly. It forms when nitrogen oxides (NOₓ), from vehicle engines, power plants and industry, react with volatile organic compounds (VOCs), from gasoline vapour, paints, solvents and even trees, in the presence of sunlight. That is why ozone levels peak on hot, sunny, still summer afternoons. Smog is the hazy mix of ground-level ozone and fine particulate matter (tiny particles called PM2.5), often with other pollutants. It irritates the eyes and lungs, worsens asthma and heart disease, and damages crops and forests. Air pollution also crosses borders: much of southern Ontario's summer smog has historically blown in from industrial regions of the United States.

• Ontario closed its last coal-fired power plant in 2014. Together with cleaner vehicles and lower emissions in the United States, this helped cut the number of smog-advisory days from 53 in 2005 to none in 2014.
• The Air Quality Health Index (AQHI) reports the health risk from a mix of air pollutants on a scale from 1 (low risk) to 10 and above (very high risk), with advice for people with lung or heart conditions.
• Wildfire smoke is now a major cause of poor air quality: during the record 2023 fire season, smoke brought some of the worst air-quality readings ever recorded to Ontario cities.
• As Toronto's medical officer of health, Dr. Sheela Basrur led studies estimating how many premature deaths and hospital admissions air pollution caused in the city each year, which helped push for cleaner air.