Burning wood - not as cosy as it seems


As temperatures drop over winter months, many Kiwis turn to their fireplaces to heat their homes. However, most of us are not fully aware of the immense impact that wood burning can have on people and the environment.

Dr Ian Longley, a NIWA Air Quality Scientist, discusses wood burning, smog and its impact on air quality.

When wood isn't burned properly it creates particle-filled smoke. [Photo: Gregory Dubus]

What are the rules for burning wood in winter?

There are rules at a national level, but also more stringent rules in certain regions and towns which have a worse air quality record.

At a national level, all new wood burners installed after 1 September 2005 on properties of less than two hectares, must have a particle emission of less than 1.5 grams per kilogram of dry wood burnt. They must also have a thermal efficiency of greater than 65%, and the burning of treated timber is forbidden. Open fires are outside the national rules; however, a number of regional councils have rules about open fires in their regional plan.

The Ministry for the Environment provides a national list of authorised wood burners that meet this standard. Whether wood burners not meeting this standard are permitted depends upon local and regional rules.

How does burning wood affect air quality?

Although wood is a very convenient source of fuel, it is not a very pure or efficient one. Unlike fuel oil, or especially gas, much of the fuel isn’t burned properly and creates a tremendous amount of smoke.

Smoke from a single agricultural fire can fill a whole valley. Smoke from home fires in winter can be smelt across our towns, causing irritation and annoyance to some, reducing visibility, and frequently causing air quality to exceed the National Environmental Standard.

What is the main source of ‘smog’ in New Zealand?

‘Smog’ is a term describing the combination of smoke and fog. Although we don’t monitor smog directly, the levels of smoke are indicated by networks of air quality monitors operated by regional councils. The monitors measure levels of ‘PM10’, or ‘particulate matter smaller than 10 microns (one hundredth of a millimetre).

Extensive monitoring and research over the last two decades have shown that smoke from home heating (mainly wood, but also coal) is the largest single contributor to PM10 levels across the country. In some smaller towns home heating contributes more than all other sources combined.

What weather conditions contribute to smog?

The weather conditions that lead to high levels of smoke are broadly the same as those that lead to fog – low temperatures, calm winds and clear skies. These conditions provide a combination of high demand for heating (which may lead to people lighting fires early, or keeping them burning for longer, or using fires to supplement regular heating) and poor dispersion of the smoke that is emitted.

Clear skies allow heat to rapidly radiate away from the earth’s surface, causing an ‘inversion’ (where the normal gradient of temperatures reducing with height is ‘inverted’) which suppresses the ventilation of damp or polluted air, keeping it trapped near the surface, leading to fog or a thick layer of smoke. In valleys and basins this can be worsened by extra sheltering from the wind, preventing the build-up from being released, and cool breezes flowing off the cold hills strengthening the inversion, further preventing the escape of pollutants.

What are the unhealthy compounds in smog?

Smoke generated from the burning of wood contains a complex cocktail of soot and organic compounds, some of which are toxic or carcinogenic. It also contains the odourless gas carbon monoxide, which (at high levels) can cause headaches and nausea.

However, in addition to these specific compounds, many people can have an adverse reaction to the particles themselves, regardless of their chemical composition. Particles present in wood smoke have been known to trigger asthmatic reactions, enter the bloodstream and worsen cardiovascular disease.

Alexandra. [Photo: Stuart Mackay]