Canadian Environmental Sustainability Indicators

'Emissions of some substances can harm human health, wildlife and biological diversity. For example, small particles of toxic metals in the air can be inhaled. They can also settle on the ground and in water. There, they can enter the food web and build up in the tissues of living organisms. Some of these substances can also travel great distances by air. Exposure to these substances, even in small amounts, can be hazardous to both humans and wildlife. These indicators track human-related emissions of mercury, lead and cadmium'--Emissions of harmful substances to air indicator, p. 5.