Quick Dips

Our Deadly Air?

The Canterbury Regional Council claims that in some years up to 60 deaths are brought on by PM10 (particles slimmer than 10 microns) in Christchurch air, and that in bad periods 90% of this comes from home fires and 38% from coal burned on them. It is promoting fire bans on the basis of the solid scientific evidence it says it has to back these claims.

There are about 2,500 deaths in Christchurch each year, of which about 250 are from respiratory failure. One might then expect that respiratory deaths might be about 25% higher in Christchurch than in well-ventilated Wellington. In fact, in the years 1992-95, respiratory deaths were 9.8% of all deaths in Christchurch, compared with 11.2% in Wellington. The CRC refuses to consider this sort of evidence. It says it is irrelevant. It is certainly inconvenient!

The estimates of the amounts of PM10 coming from home fires are curiously derived. They come from a NIWA report to the CRC "Christchurch inventory of total emissions", R97/7. A home survey by telephone, not checked by any other method, provided the basic data. The authors say that there is a good fit between their estimates and the concentrations of pollutants measured on the air quality monitoring instruments. Indeed, they did get a good fit for PM10, but to manage this they assumed that the pollution ceiling did not drop below 25 metres. They say they chose this height because they then got a good fit between their estimates and the records!

On smoggy nights, recorded air temperatures are often 5-6^o higher at 10 metres than at ground level, indicating a pronounced inversion layer effect down to this level. It is this inversion layer which prevents the pollution from escaping. The cover photograph on the October issue shows the ceiling just above house level, probably below six or so metres. If this height is used to calculate their fit between estimate and actual, the answer is then out by a factor of 25/6.

Nitrogen oxides come mostly from motor traffic. According to the inventory estimates, emissions of PM10 and nitrogen oxides are about the same in the smog-generating hours. In fact, during these hours NO_x concentrations are 3-4 times higher than concentrations of PM10. CO concentrations are estimated to be about 10 times higher than PM10; they are actually about 60 times higher. The authors dismiss these gross discrepancies as "minor uncertainties".

So I read with interest the article by Janine Clemons rhetorically titled "Is our air deadly?". She may have evidence that breathing Christchurch air is killing large numbers of people each year. I did not find it in her article, though I gathered that she believes that it does. Are her beliefs more evidentially based than Dr John Carter's belief that hands heal?

The claims are serious. They deserve serious science. So do environmental issues generally. So does the city which nurtured Rutherford and Popper. A desire to be clean and green, though laudable, is not enough.

Pat Palmer, Christchurch