4th March 2011, Volume 124 Number 1329

Peter Moller


Severe fogs in London in 1952 caused the deaths of an extra 4000 to 12000 people.1 Because of calm weather, the black smoke and sulphur dioxide associated with the burning of coal reached greatly increased levels. Legislation in relation to fossil fuels overcame much of this sort of pollution. Since then, photochemical smog has become more of a problem in many cities and is associated with increased motor traffic and high concentrations of ozone and nitrogen oxides. Associated with this, fine particulate matter (PM10 - particles with diameter less than 10 microns, and PM2.5) has become of concern.


Numerous studies have shown an association between daily mortality and PM10 levels.2Estimates of increased mortality associated with PM10 have generally been based on the air pollution levels in an area, correlated with mortality statistics for that area. Sometimes attempts had been made to control for other risk factors such as smoking, low income and education. There has been little attention directed to climatic conditions.

The APHEA study in Europe showed that all-cause daily mortality increased by 0.6% for each 10 mcg/m3 increase in PM10. The NMMAPS study in the USA found the increase to be 0.5%. Some of the studies which averaged pollution levels over long periods of time and compare the death rates in two different communities assume that measurements from a central site are indicative of the exposure for a wider population.

The results of this have been conflicting, but where excess mortality occurred it was due mainly to cardiovascular and respiratory disorders. Despite this, a review of European studies in adults3 has not provided consistent evidence of an association between PM exposure and chronic bronchitis or asthma. No consistent associations between lung function and 24-hour average particle numbers or particle mass concentrations were found in panels of patients with mild to moderate COPD or asthma.4

There are difficulties in interpreting the findings from studies which describe extra deaths over a short-term in relation to pollution levels .5 Such studies can only identify the acute effect of pollution. Part of the increase in mortality may be due to the death of individuals who would have died only a few days later from their serious illness. This would overestimate the impact of air pollution on health. Focussing on a single pollutant may overestimate its impact since the levels of different pollutants may be linked, as they are, for example, with traffic emissions. Estimates of premature deaths due to PM10 air pollution must at present be considered speculative.

In relation to PM10 there has been a lack of specificity. It has been assumed that all PM10 will affect health in much the same way. The WHO guidelines6 state: “ Although few epidemiological studies have compared the relative toxicity of the products of fossil fuel and biomass combustion, similar effect estimates are found for a wide range of cities in both developed and developing countries. It is, therefore, reasonable to assume that the health effects of PM2.5 from both of these sources are broadly the same.” Is there sufficient evidence to support such an assumption?

PM10 is not a single entity

Particulate matter is a mixture of solid and liquid particles suspended in the air.

In the London deaths from smog, particles associated with sulphur dioxide clearly played a critical role. Today, ozone and nitrogen dioxide along with particulates are the potential pollutants of greatest importance. The major contributor to nitrogen dioxide in the atmosphere is the burning of fossil fuels for heating, power generation, and powering motor vehicles.

There is a background level of PM10 in any location. This may come from a number of natural environmental sources. The Health and Air Pollution in New Zealand final report7states: “The research community has not yet resolved the question of whether background sources have the same epidemiological effect as anthropogenic combustion sources.” Despite this “ the regulations require councils to mitigate various sources of PM10 in order to achieve the standards...if the amount of PM10 due to background sources is a significant fraction of the total then other sources may need to be mitigated more heavily.” This confirms a lack of discrimination between possibly harmful and innocuous contributors to the PM10 level.

In coastal areas much of the PM10 is derived from salt spray. It is hard to see that this is a hazard. Indeed, in Bad Reichenhall, Bavaria there is an inhalatorium—two 175 m corridors with 14-metre high bundles of twigs through which brine trickles. The salt in the air is claimed to help respiratory disorders, and an hour a day walking along the walls is recommended.

This lack of discrimination between various PM10 sources and their association with cardio-respiratory disorders raises several questions. Is particulate matter in itself a serious hazard or is it a surrogate measure? Can particulates increase the ill effects of active pollutants? Brunekreef and Holgate1 feel that the most complex question is which particulate matter components or attributes are most important in determining health effects. They quote evidence that individuals who live on the main roads in Amsterdam have a much higher relative risk of death than people who live away from the main roads, when analysed with data from the same background air pollution monitoring station. This suggests that particles derived from motor traffic are particularly important.

In his Cabinet paper “Review of the PM10 Air Quality Standards” the Minister for the Environment8 states 'Most PM10 in New Zealand comes from burning solid fuels (i.e. coal and wood) for home heating. This, along with more frequent settled weather conditions during winter, is why most peak PM10 levels occur during this time of year. However, the PM10 profile changes during summer when transport and industry emissions become the major sources of emissions.”

The finding by Fisher et al7 that the effect of PM10 was much less in winter (0.9% per 10 mcg/m3) than in summer-3.9% suggests that either the source of PM10 is critical to its pathogenicity or that atmospheric conditions such as those on sunny hot days which lead to photochemical reactions with ozone and NO2 are important. Similarly, in Belgium,9 there is stronger association between daily mortality and fine particulates in summer than in winter. The percentage increase in mortality on days in the highest season-specific PM10 quartile versus the lowest season-specific PM10 quartile were 7.8% in summer and only 1.4% in winter.

The burning of wood produces particulates but whether these in themselves are a health problem is uncertain. Clearly more knowledge of the relationship between the structure and composition of particulates and their pathogenicity is needed to provide scientifically sound guidelines. Because the science is complicated, the public have tended to acquiesce to the current regulations, assuming they represent the best expert advice.

Restrictions in Christchurch

Because Christchurch frequently has a winter inversion layer, with consequent build-up in pollutants, it has been the most carefully studied site in New Zealand. It is claimed by Environment Canterbury that 80% of PM10 in Christchurch comes from wood or coal burners and open fires.10

In the UK, road transport contributes 83%.11 There have been three papers relating to Christchurch: Hales,12 Harre,13 McGowan.14 In these, associations of increased mortality with PM10 levels are of low statistical significance and hence cause and effect are speculative. The structure and composition of the particulates in Christchurch have not been adequately analysed and their potential pathogenicity is unknown. These inadequate data have, nevertheless, been used in public relations campaigns.

Excess deaths

Janke et al5 calculate that a reduction of their sample mean PM10 from 24.7 to 20.0 mcg/m3would be associated with 8.4 fewer deaths per 100,000 population, or 4200 deaths per annum for England. The annual average PM10 level in Canterbury between 2005 and 2008 varied from 18 to 21 mcg/m3.15

Capital cities in Europe2 typically have mean PM10 levels around 20 to 40 mcg/m3.If the level of mortality due to PM10 suggested by Janke is true, it is nevertheless small compared to the well-documented excess deaths in the winter due to cold.

The importance of cold surroundings

In the United Kingdom there are 25 to 50,000 excess deaths in the winter.16 In New Zealand there are 1600 and this level is 2% higher than the mean increased mortality rates for 14 European countries in the winter.17

There is strong biological plausibility that cold conditions lead to viral infections, secondary bacterial infections such as bronchitis and pneumonia, with “decompensation” in the elderly leading to cardiovascular and respiratory failure. At the other end of the age scale, infants can be at grave risk from these respiratory infections.

Cold is also an important trigger for asthma. Chung et al18 found that 70% of patients claimed cold air as a key trigger for an asthma attack. Deaths from ischaemic heart disease, the biggest single cause of excess mortality in winter take place hours or a day or two after exposure to cold suggesting they may result from thrombosis associated with cold exposure.19

In some winters such mortality has been as much as 70% higher than in the summers.20These important statistics demand that any action which is taken in relation to possible pollutants in the atmosphere must not jeopardise the ability of people to keep warm in the winter. It is probable that many deaths linked by the statistics to particulates are directly related to cold conditions.

The standards and regulations for PM10.

The WHO6 has recommended a mean annual level of 20 mcg/m3 for PM10 and a target for 24-hour concentrations of 50 mcg/m3. It also recommended that the annual average should take precedence over the 24-hour average, since at low levels there is less concern about episodic excursions. For some reason the Ministry for the Environment decided against this advice and determined that just one average in excess of 50 mcg/m3 on one day should be the point for sanctions to apply. This has led Environment Canterbury to ban open fires and require the replacement of older wood burners.

Nitrogen dioxide, sulphur dioxide and ozone

Given that the burning of coal is now banned except in well-controlled and monitored industrial furnaces, sulphur oxides might not have been a problem, but an increase in the number of diesel vehicles and the sulphur level in the diesel in New Zealand may increase sulphate-containing PM10.

Nitrogen oxides and ozone are very important and have been shown in many papers to be associated with exacerbation of respiratory disorders. Because of this, in London, measures have been introduced to decrease traffic emissions in the city. These include a restriction on the number vehicles entering central London each day and discouragement of the most polluting heavy goods vehicles from entering. Whilst we do not yet have this level of problem in New Zealand, it would be sensible for the long-term planning of air pollution to address transport planning.

Nitrogen dioxide is not only an atmospheric problem in cities, but also affects internal air quality in homes. Spengler et al21 found NO2 levels inside the kitchens of 112 homes with gas stoves averaged about 50 mcg /m3 higher and bedroom levels were about 30 mcg/mhigher than outdoor levels.

Cooking with gas is identified as the principal source for high concentrations, although gas hot water heaters, gas clothes driers, and gas and kerosene space heaters may contribute to elevated indoor levels. Since we spend more than 80% of our time indoors,1 the indoor environment may be even more important than outdoor pollution.

The burning of wood in fireplaces and conventional wood stoves emits 10 times more primary PM2.5 than nitrogen oxide per unit of wood burned. Therefore, the impact of nitrogen oxide-derived secondary PM2.5 from wood smoke is expected to be small.22 This may add to the argument in favour of electricity and wood for indoor heating.

Social and economic consequences

In recent years there have been several occasions when there was concern about the adequacy of storage levels in our lakes. Given that there will always be some uncertainty about the adequacy of hydro-power we need to retain flexibility for domestic heating in the winter to ensure warmth in our homes. Conservation in the use of electricity will also help to avoid risk to our energy security.

If cities and communities phase out the burning of wood then there will be greater pressure on the supply of electricity and LPG. To meet the increased electricity demand the cost of electricity will increase through the need for more generation and line capacity. The cost of LPG is likely to increase over time.

The installation of new wood burners and alternative heating sources entail significant individual costs and, if subsidised, community cost. Improvement of the insulation in older houses will reduce the demand for energy input, reduce daily heating costs and contribute to our long-term energy security. There is both an individual and subsidy cost.

Wood is a cheap heating fuel which is important to those on lower incomes. To reduce its availability, or to increase the costs involved in continuing to use it, will have significant repercussions on the less affluent. The communal aspect of sitting round a fire will also be lost. The social consequences of this have not been addressed.

The Minister for the Environment has raised the problematic implications of the current regulations for employment.8 “The non-complying airsheds beyond 2013 would be prohibited from approving any renewal of existing air discharge consent or new consents, with a considerable impact on employment. It has been estimated...that 233 consents employing approximately 17,600 people would be affected.” This illustrates the need for a broad appreciation of the implications of regulations and the importance of reliable scientific evidence to underpin sound policy decisions. To cope with the difficulty the Minister has suggested that three days in excess of 50 mcg/m3 should now be allowed.

The Updated Users Guide to Resource Management Regulations 200423 lists the priorities that should be considered before imposing regional policies: impact on emission reduction, feasibility, cost, benefits, resources, equity, fairness and/or parity, external influences such as security of supply of energy. Cost and equity will also be affected by the Emissions Trading Scheme.24 As the increased cost of carbon is passed on through rising prices of products and services it will bear most heavily on those with low incomes, in this case through the cost of electricity and LPG. Because wood is a carbon-neutral energy source its use may mitigate this and reduce the need for more expensive coal-fired electricity production.

Dhar et al have stated: “In New Zealand, low-income households already spend a higher proportion of their income than high-income households on non-discretionary carbon-related expenses such as household fuel and power. Households in the lowest income quintile now spend 9.7% of their income on household fuel and power compared to 7.1% in 2004. Even without an Emissions Trading Scheme or carbon taxation, the pressures on low-income households will continue if the price of energy rises. Fuel poverty—defined as houses spending more than 10% of their income on fuel use to heat their home to an adequate standard of warmth—already affects between 10 and 15% of all households in New Zealand.”24

They also suggested that the negative impacts of carbon taxation policies could be softened or even avoided by revenue recycling. For instance, revenue generated by carbon-charging might be directed to housing insulation for those in higher deprivation levels.


The preoccupation with PM10 levels on single days rather than the average annual level, and the apparent lack of concern for affordable fuel has been a misjudgement. The adverse health effects of cold conditions are well established. Adequate domestic heating is important to prevent excess morbidity and mortality in the winter. This is being undermined by the standards and regulations to control PM10 in the atmosphere imposed by the Ministry for the Environment.

These regulations are not adequately supported by the scientific evidence and they should be suspended. The more flexible arrangements suggested by the Technical Advisory Group to the Minister for the Environment are inadequate. They maintain an attitude to PM10 based on assumptions and estimations criticised above.

Until the science in relation to PM10 is clarified, restrictions on the use of wood for domestic heating should be minimised, to allow citizens to be warm at reasonable cost.

Restricting the use of a carbon neutral, renewable source of energy for home heating, such as wood, should not be contemplated without the most compelling scientific evidence against its use. Such proposals should also take account of overall energy security.

The equity of heating decisions for those with lower incomes needs to be carefully considered.


There is excess mortality in the winter. To minimise this it is important that an adequate indoor temperature is maintained and this is dependent on affordable energy supplies. Standards adopted by the Ministry for the Environment in relation to levels of small particles (PM10) in the air and the Regulations to enforce their implementation are based on inadequate scientific evidence. They are likely to make heating less affordable and have a negative net effect rather than a positive one on general health. Whilst the attainment and maintenance of clean air is laudable, regulations should be based on sound scientific evidence. The costs, benefits, and equity for individuals need careful consideration, as do the implications for energy security.

Author Information

Peter W Moller, Rheumatologist, Christchurch


I thank The Association for Independent Research, which has continued to seek a rational approach to PM10, as well as Professors E J Begg and M G Nicholls for their critical appraisals.


: P W Moller, 17 Rhodes St, Christchurch 8014, New Zealand.

Correspondence Email


Competing Interests



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