Home heating: what’s the problem?

By Guy Coulson

Use of solid fuel, mostly wood but also coal, particularly in some South Island towns, is a major source of night-time airborne pollution in many places in New Zealand. On cold, still winter nights dispersion is poor and emissions from domestic solid-fuel heating can build up, leading to high concentrations of pollutants. The problem can be exacerbated by terrain where pollutants collect in valleys and low lying areas. Globally, the problems of woodburning are similar to those in NZ. In Alpine valleys across Europe and North America woodsmoke collects, leading to high concentrations of particles in particular.

The 2013 census found that 546,000 dwellings (36 percent of all dwellings) in New Zealand used wood for heating, a similar number to small northern European countries such as Norway or Denmark. A 2005 survey by MfE estimated that these fires could burn more than 13,000 tonnes of wood a day during winter.

The main products of combustion are Carbon Dioxide ($CO_2$) and water but alongside these, the emissions from a wood burner consist of a complex mixture of particles and gasses including carbon monoxide ($CO$), nitrogen oxide ($NO$), sulphur dioxide ($SO_2$) and several hundred different organic compounds. Particles generated by wood burning consist of inorganic ash, soot, and condensed organic compounds. $CO$, soot, and organic compounds are products of incomplete combustion; the quantities and composition are dependent on combustion efficiency.

Exposure to smoke from residential wood-burning has been associated with increased respiratory symptoms in children and adults, decreased lung function in children and increased hospitalisations. Wood smoke has also been classified as “Probably carcinogenic” by the International Agency for Research on Cancer. At least 26 hazardous air pollutants are known to be present in woodsmoke including nine carcinogenic ones.

Marsterton, winter 2006. (Photo GWRC). This photo shows how woodsmoke can hang around close to its origin during times of low wind

Around the World, the focus of research and regulation as far as woodburners are concerned has been on particles as the chief cause for concern and the situation is no different in New Zealand. Most breaches of the National Environmental Standard (NES) for PM₁₀ are attributed to domestic heating (other causes are fireworks, sea salt, Australian desert sand and volcanic ash). The NES proscribes a limit of 50 micrograms ($\mu g$) of PM₁₀ per cubic metre ($m^3$) of air, measured as a 24 hr average (midnight to midnight). Concentrations of more than 200 $/mu g/m^3$ having been recorded in some towns in New Zealand (the maximum recorded value was 283 $/mu g/m^3$ in Christchurch in 1994). Several towns and cities in New Zealand still regularly record concentrations of over 100 $/mu g/m^3$ on a winter’s night and in 2012, 50% of all monitoring sites recorded an exceedance of the NES.

So, woodsmoke is known to be harmful but it’s not clear how much you need to breathe in to cause you personally any ill effects. Most of the medical evidence is based on long-term exposure (based on annual average concentrations and large populations, assuming everyone is exposed to the same amount all the time for many years) or by subjecting healthy volunteers to rather large doses for short periods, around half an hour. In New Zealand you tend to be exposed to higher concentrations of woodsmoke for just two or three months a year and then nothing for the rest of the year. If you’re perfectly healthy, it may not do you too much long term harm but if you have asthma, a heart condition or are otherwise predisposed to be sensitive to the effects then it may be making things considerably worse. This research is amongst other things, aimed at helping to answer some of the questions about how different exposures to woodsmoke affect different people.