Where am I exposed to air pollutants in my daily life?

Your exposure to air pollutants may show surprising fluctuations throughout your day. Even within a small city, there can be tremendous variation in local air quality.
Given the following list, hypothesize where in your day you might find the greatest and poorest air quality.

A.      Sitting in a car with the windows down at an intersection
B.      Spending time in your home
C.      Going for a walk in a quiet neighbourhood
D.      In a parking garage with a car engine running
E.       Working in a building with the widows open next to a construction site
F.       Biking on a city street in traffic on a sunny day

To be more informed about where air pollutant levels could be highest in your daily life, learn about the sources of the air pollutants highlighted in this resource.


It is important to clarify that many of the pollutants we are considering not only are produced by human activities (anthropogenic sources) but also in natural processes. Although natural sources have always contributed to the production of low amounts of these substances in the atmosphere, human activities can sometimes increase the amount of pollutants to dangerous levels.

Another important detail to note is that certain pollutants can react with other substances in the air to form other pollutants. For example, nitrogen oxides (NOx) and sulfur dioxide (SO2) are involved in complex reactions to increase the atmospheric level of particulate matter (PM). Primary pollutants are those that are emitted directly from a particular source, whereas secondary pollutants result from chemical reactions involving primary pollutants.

Particulate matter (PM)

Particulate matter is considered both a primary pollutant and a secondary pollutant. Primary particulate matter can originate from both natural phenomena (such as wildfires or blowing dust) and anthropogenic activities (such as coal-burning and vehicle engines).Secondary particulate matter forms as a result of chemical reactions between existing molecules in the atmosphere such as nitrogen oxides, sulfur oxides, and ammonia.

Nitrogen oxides (NOx)

Nitric oxide (NO) forms when N2 and O2 react at very high temperatures. Nitric oxide formation occurs naturally in the presence of lightening. Anthropogenic sources of nitric oxide primarily include vehicle engines and coal-fired power plants (since they provide high enough temperatures for N2 and O2 to react). Nitrogen dioxide can also be produced from combustion reactions in vehicle engines, but most of it is formed from existing nitric oxide.Since the levels of nitric oxide and nitrogen oxide are closely related, they are often referred to collectively as “nitrogen oxides” (NOx).

Ground-level ozone (O3)

Tropospheric ozone* is considered a secondary pollutant, and is a major constituent of photochemical smog, which can sometimes be noticed visually. Photochemical smog forms from a reaction between nitric oxide, volatile organic compounds (VOCs) and oxygen in the presence of sunlight. NO and VOCs can both be emitted from vehicle exhaust. As a result, ozone forms most readily when there is dense traffic, sunlight, and high enough atmospheric temperatures.

*It is important to distinguish tropospheric ozone from stratospheric ozone. In the troposphere, ozone is a harmful pollutant but in the stratosphere, ozone plays an important role in shielding earth from damaging levels of UV radiation.

Sulfur dioxide (SO2)

The largest source of sulfur dioxide is the burning of fossil fuels for power generation, transportation, and indoor heating or cooking. The high levels of sulfur dioxide during in “The Great Smog” in London in 1952 were largely attributed to excessive coal burning. (It is estimated that the mix of air pollutants, including PM and SO2, in the Great Smog were responsible for over 12 000 deaths.)

Carbon monoxide (CO)

Carbon monoxide is released into the air by the incomplete combustion of fuel – such as coal, wood, natural gas, kerosene and petrol. Anthropogenic emissions account for about two-thirds of all atmospheric carbon monoxide levels.  The primary source of outdoor carbon monoxide is automobile emissions. As a result, the highest levels of carbon monoxide in many cities tend to occur on weekdays during peak commuting hours.  Although outdoor carbon monoxide exposure does pose concern, carbon monoxide exposure indoors is far more alarming. Inside enclosed spaces, carbon monoxide gas cannot dissipate as readily as it can outdoors. Consequentially, carbon monoxide concentrations may be much higher in enclosed areas.  For example, the amount of carbon monoxide may reach dangerous levels when a car engine is producing exhaust in a closed garage. Tobacco smoke can also cause elevated carbon monoxide levels. Common indoor sources of carbon monoxide include unventilated gas appliances (such as space heaters or gas-burning stoves), and wood-burning stoves and fire places. Leaking chimneys and furnaces can also be sources for human exposure to CO.

Lead (Pb)

The main sources of lead air emissions today are lead smelters, lead-acid battery manufacturing sites, and waste incinerators. However, prior to the 1970s, the main source of lead air pollution resulted from automobile emissions powered with leaded gas. Leaded paint was also a source of lead pollution. However, since leaded gas and leaded paint have been gradually phased out, lead air pollution has improved substantially.