Continued from PART 1.
In Part 1 we saw that it is untrue that air pollution causes 28,000 deaths each year in otherwise healthy people.
But could these deaths be ‘linked to’ air pollution? Let’s look at the adverse effects of air pollution.
DEFRA list the effects of very high levels of some key pollutants (link):
- NO2, SO2, O3 (Ozone): These gases irritate the airways of the lungs, increasing the symptoms of those suffering from lung diseases
- Particles: Fine particles can be carried deep into the lungs where they can cause inflammation and a worsening of heart and lung diseases
- CO (Carbon Monoxide):This gas prevents the uptake of oxygen by the blood. This can lead to a significant reduction in the supply of oxygen to the heart, particularly in people suffering from heart disease
So all these pollutants can be expected to harm us. But how much harm they do? This question – the quantitative expression of the harm caused by air pollution – is at the heart of this issue.
Broadly speaking, we can expect short-term and long-term effects. However, it is possible for air pollution to have both kinds of effects in the same person.
Short Term Effects
The US National Institute of Health (NIH) host this 1966 paper by McCarroll and Bradley looking for excess mortality coincident with air pollution events in New York City in 1962.The paper is notable for its readability.
McCarroll & Bradley analysed 3 pollution ‘events’ and the figure at the top of the page summarises the general nature of their analysis: they note excess mortality coincident with air pollution events, and a reduction in mortality for subsequent days. They note specifically that the drop is “…never of sufficient degree to compensate for the excess of deaths on the preceding day.”
So the general mechanism in action here is that air pollution has ‘brought forward’ people’s deaths: some by just a day or so, but others by unknown periods of time – probably weeks to months. These would be people with an underlying medical condition which itself may possibly be a long term effect of air pollution
Long Term Effects
Long-term effects are more difficult to assess since we do not have identical healthy populations that differ only in their long-term exposure to pollutants. However such effects can be estimated by so-called cohort studies.
So by analyses similar to McCarroll & Bradley and by cohort studies we can begin to estimate how much air pollution causes how much excess mortality.
Short-term exposure to air pollution appears to bring forward some deaths – the actual number is unknown.
Long-term exposure to air pollution also appears to also shorten life.
For the UK population, a committee (COMEAP) estimates this to amount to 350,000 person years per year. What?
- For a population of 60 million this amounts to about 2.1 days per person, per year of life.
- So if the ‘natural lifetime’ of a person born now is 80 years, then exposure to typical air pollution for their lifetime is estimated to shorten their lives.
- The shortening is calculated to be 80 years x 2..1 days ≈ 6 months less than they might otherwise have lived.
So now I think I understand the nature of this excess mortality. There are three components
- Air pollution has adverse physiological effects,
- People susceptible to heart attacks or with respiratory diseases (from other causes) can be brought into a cycle of distress which brings forward their death.
- Additionally chronic prolonged exposure to air pollution can induce respiratory complaints that may make people susceptible to acute air pollution events.
Estimating and Communicating these effects
In PART 3 we will see how that estimate is made, and you can judge for yourself whether to be alarmed.
Tags: Air pollution