Particulate matter (PM2.5)

Particular matter refers to a complex mix of tiny solid and liquid particles in the air, such as dust, dirt, soot, and smoke. Some particles are as small as 2.5μm or less. The unit "μm" is a micrometer (a millionth of a meter). To put this in perspective, a human hair is about 60μm in diameter. While individual particles less than 2.5μm are impossible to see with the naked eye, many of them in the air at the same time causes reduced visibility or haze. The smallest particles are able to bypass protective filtering mechanisms, and penetrate deeply into the lungs. Once inside the respiratory system, these particles can lead to respiratory damage and other health complications. Exposure to particulate matter increases the risk of developing heart diseases, respiratory diseases, and lung cancer, and continual exposure to PM in places like severely polluted large cities is linked to increases in serious health effects and mortality. Particulate matter can also aggravate pre-existing cardiovascular or respiratory diseases. 

Sulfur dioxide (SO2)

Sulfur dioxide is a colourless gas that is produced during the combustion of fuels and other materials that have low levels of sulfur-containing compounds.  SO2 has a sharp, unpleasant smell. At high levels, exposure to sulfur dioxide can cause death.  At much lower levels, it leads to eye and respiratory irritation, and can increase the likelihood of cardiovascular and respiratory diseases. Individuals with existing asthma are particularly sensitive to the effects of sulfur dioxide. Exposure to sulfur dioxide has also shown to increase susceptibility to respiratory infections and has been linked to chronic bronchitis. The amount of sulfur in gasoline is now regulated in many countries to reduce the levels of SO2 in urban air.

Nitogen dioxide (NO2)

Nitrogen dioxide is formed from the high temperature reaction of nitrogen gas (N2) in the air with oxygen gas (O2) as a side reaction of the combustion of fuels. Nitrogen dioxide is a reddish-brown gas with a sharp, unpleasant smell, if concentrations are high enough. However, at very low concentrations, the colour or smell of nitrogen dioxide may not be noticed. Exposure to nitrogen dioxide increases the likelihood of developing respiratory diseases because it inflames the lining of the respiratory tract. Nitrogen dioxide also aggravates existing respiratory diseases (particularly asthma). 

Ground-level ozone (O3)

Ozone is a colourless gas with a “clean” smell – you may have experienced it as the “clean” odour in the air following a summer thunder and lighting storm. It is important to distinguish ground-level ozone (tropospheric ozone) from stratospheric ozone. High up in our stratosphere, ozone is formed by reactions involving oxygen molecules and light from the sun.  In the stratosphere, ozone protects life on Earth from getting too much UV radiation from the sun. The ozone in the stratosphere is formed naturally and is beneficial to life on Earth.  

Unlike stratospheric ozone, most ozone in the lower troposphere comes from human activity – it is formed as part of a cycle of reactions of other air pollutants that lead to the production of smog. Ground-level ozone poses a concern to our health, and it is one of the pollutants of concern when a “smog alert” is issued in a heavily polluted city. When ozone enters the lungs, it can cause coughing, irritation of the airways, and result in an increased susceptibility to respiratory infections. Ozone can also aggravate existing lung diseases such as asthma, emphysema, and chronic bronchitis. 

Carbon monoxide (CO)

Carbon monoxide gas results from the combustion of fuels and other carbon-containing substances under conditions in which there is not enough oxygen present to produce carbon dioxide. Because CO is a colourless, odourless gas, small concentrations of carbon monoxide can easily go unnoticed without proper detection equipment. However, even very small amounts of carbon monoxide pollution can have a significant impact on human health. Carbon monoxide reduces the amount of oxygen that circulates in our blood because it readily binds to hemoglobin (the protein in our blood that carries oxygen), reducing the amount of hemoglobin available to distribute oxygen to the cells in our bodies. Decreased oxygen in the blood can have a wide range of effects on the body. If carbon monoxide exposure is minimal, you might experience tension-type headaches, fatigue, difficulty concentrating, or nausea. High levels of carbon monoxide exposure can cause much more serious symptoms and can even be fatal. 

Lead (Pb)

Lead is a metal that is still found in urban air  as a constituent of particulate matter, as a result of using lead additives in fuels for decades around the world. After exposure, lead makes its way into the bloodstream, where it circulates around the body. Eventually, lead is stored in the bones. Lead exposure affects almost every part of the body in people of all ages. However, fetuses and very young children are the most sensitive to the effects of lead. Even very minimal amounts of lead can result in learning deficits, behavioural problems, and delayed growth in young children. Lead exposure in pregnant women is especially concerning because lead can cross the placental barrier and affect the fetus.  In adults, mild lead exposure can cause hypertension, decreased kidney function, and reproductive problems. 



To learn about the World Health Organization guidelines for air pollutants, visit