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Facts About...


What is smog?

Smog is a noxious mixture of airborne pollutants. The main components of smog are ground-level ozone and fine particles known as particulate matter. Pollution from industrial sources and vehicle exhausts are the primary contributors to smog. High levels of smog are typically associated with the summer. However, the smog problem actually occurs throughout the year, with winter smog (due to particulate matter contributions rather than ozone) being a serious concern when stagnant air causes a build up of pollutants in the air.

What is ground-level ozone?

Ground-level ozone is produced by a reaction in the lower atmosphere between nitrogen oxides (NOx), volatile organic compounds (VOC), sunlight and warm temperatures. As a result, it is known as a summertime air pollutant. Nitrogen oxides are produced when fossil fuels such as gasoline, natural gas, heating oil and coal are burned. Volatile organic compounds originate mainly from gasoline combustion and from the evaporation of liquid fuels and solvents. Ground-level ozone shouldn’t be confused with the ozone layer high above the Earth that protects us from UV rays.

What is particulate matter?

Particulate matter (PM) consists of airborne particles in solid or liquid form. Generally, any activity which involves burning of materials or any dust generating activities are sources of PM. The formation of PM also results from a series of chemical and physical reactions involving different gases, such as sulphur and nitrogen oxides, as well as ammonia reacting to form sulphate, nitrate and ammonium particulate matter. The size of PM particles largely determines the extent of environmental and health damage caused. Therefore PM is identified by its size:
  • Particulate matter less than 10 microns (PM10), (about 1/10th the thickness of standard copy paper)
  • Particulate matter less than 2.5 microns (PM2.5) (about the thickness of a garden spider’s web)

What are the potential health effects of smog?

Smog can irritate your eyes, nose and throat and may aggravate existing heart and lung problems. In exceptional cases it may result in a premature death. Many of these problems are more common in seniors, making them more likely to experience the negative effects of smog. Children may be more sensitive to the effects of smog because children breathe in a greater volume of air than adults relative to their body size and their respiratory systems are still developing. Even healthy young adults breathe less well on days when the air is heavily polluted.

The health effects of ground-level ozone and PM is also cause for concern. Some studies suggest that long-term regular exposure to PM can increase the risk of premature death and lung disease including cancer. Studies on ozone show that once it gets into your lungs, it can continue to cause damage even when you feel well.

How will I know what kind of day it is?

The Air Quality Health Index (AQHI) is a national tool available to help individuals protect themselves and the people they care for. The Air Quality Health Index reports air quality in relation to health on a scale from 1 to 10. The higher the reading, the greater the health risk and need to take precaution.

The AQHI helps people plan ahead by telling them the best time of the day to be physically active and when to reduce or reschedule strenuous outdoor activity. The AQHI provides health messages for both the at-risk and the general population. The at-risk group includes children, the elderly, and people with pre-existing heart and breathing problems.

When the Air Quality Health Index reaches seven or above, people with heart or breathing problems should reduce or reschedule strenuous outdoor activities. Children and the elderly should also take it easy. AQHI forecasts are posted at and readings are updated hourly.

The AQHI helps to protect our health, and also serves as a reminder of the need to protect our environment. As cars, trucks and coal-fired power plants are major sources of air pollution, reducing energy use can reduce the number of high AQHI days.

More information can be obtained from

Durham Region Health Department, Environmental Help Line 905-723-3818 or 1-888-777-9613.

April 22, 2013