Carbon monoxide (CO) is a colourless, odourless gas usually released when something is burned.
The greatest sources of CO are cars, trucks and other fossil-fuelled machinery. Breathing air with a high concentration of CO reduces the amount of oxygen coming to critical organs like the heart and brain.
At very high levels (indoors), CO can cause dizziness, confusion, unconsciousness and death. Very high levels are not likely to occur outdoors, but elevated CO levels outdoors can be of concern for people with some type of heart disease, especially in situations where the heart needs more oxygen than usual, e.g. while exercising or under increased stress. In these situations, short-term exposure to elevated CO may result in chest pain also known as angina.