Air pollution in Christchurch

Eighty percent of Christchurch’s winter air pollution comes from wood or coal burners and open fires. Only 10% comes from vehicles and 10% from industry.

The older and less efficient a fire is, the greater the quantity of emitted dangerous fine particulate emissions (known as PM₁₀). On cold, still winter nights, these particles can form into a choking, brown smog.

For up to 50 days each winter, the level of PM₁₀ particles in the air in Christchurch exceeds Ministry for the Environment guidelines. Research indicates each year this pollution is responsible for:

• serious health problems for several thousand people, such as respiratory and cardiac illness (causing them to take time off work, which affects the local economy)
• the premature deaths of more than 150 people due to respiratory or cardiac illness
• higher health costs for everyone, due to heavier demand on the health system
• a damaging effect on the public image of Christchurch.

In Christchurch, because there are hills close by, a layer of warmer air, known as an inversion layer, traps the smog down at street level. This directly affects the people who live there. Although the problem is at its worst in the evening, the smog is often clearly visible the next morning.

What is a temperature inversion?

You may think that, on a sunny day, it is the sun warming the air around you, but that’s not strictly true. What is actually happening is that the earth is absorbing the sunlight and then radiating that energy back out as heat. What this means is that normally the air is warmest at ground level and gradually gets colder in higher parts of the atmosphere. At the end of a cold calm winter’s day, the temperature of the earth can drop after the sun goes down. If the warm air is not moving because there are hills that prevent this, the air closer to the ground can be colder than a warmer layer above it and the two layers do not tend to mix. This is called temperature inversion. This effect traps any pollutants close to the ground, and you breathe these in, wherever it occurs.

Things to think about

Rights: The University of Waikato

Temperature inversion

This diagram of a temperature inversion illustrates the way in which a layer of warm air traps a layer of cool air, and with it pollution, close to the ground. Without this inversion layer pollution would be able to rise into the atmosphere and disperse.

During a temperature inversion the cold layer closest to the ground is caused by cold dense air sinking down surrounding hills underneath the less dense warm air.

Cause and effect related to human health is hard to show.

• If you were a scientist, what would do to show the link between air pollution and breathing problems?
• What things would you need to think about before setting up experiments on people?
• What other evidence could demonstrate that air pollution causes breathing problems?