Archive for May, 2015

Did 28,000 people die from air pollution in 2012? Part 3

May 28, 2015

 

COMEAP needed to estimate how many percent excess mortality would be caused by 10 micrograms per cubic metre of PM2.5 air pollution. To do this they just asked themselves what they thought the answer was. The details of this 'elicitation' process are described below.

The Committee on the Medical Effects of Air Pollutants (COMEAP) needed to estimate how many percent excess mortality would be caused by 10 micrograms per cubic metre of PM2.5 air pollution. To do this they just asked themselves what they thought the answer was. Their answers range from -1% (i.e. air pollution prolonged life) to 17% excess mortality. The details of this ‘elicitation’ process and the meaning of excess mortality are described in the text.  This graph represents their consensus view: individual estimates are shown at the end of the article. (SOURCE COMEAP 2009 Page 160.)

Continued from PART 2

In Part 1 we saw that is untrue that air pollution annually causes 28,000* deaths of otherwise healthy people.

In Part 2 we described the qualitative effects of air pollution.

We now look at how the Committee on the Medical Effects of Air Pollutants  (COMEAP) estimate the effects of air pollutants, and how they have chosen to communicate their estimates.

What I have written below is the result of staring at reading two extremely technical documents, an activity which I invite you to share:

The factual basis…

The COMEAP studied hundreds of scientific papers but singled out one for special mention: the American Cancer Study (ACS) [COMEAP 2009 Page 2: Point vii]

The ACS uses measurements of soot particles called PM2.5 as a ‘proxy’ for all the other forms of air pollution.

It does this because PM2.5 s are easy to count and the concentration of the other air pollutants is more difficult to quantify, but they are typically correlated with PM2.5s.

So we don’t know which part of the pollution is actually the active cause of any effect – a shortcoming the experts consider exhaustively in their 2009 report.

The ACS finds a correlation such that there is a 6% rise in all-cause mortality for a 10 microgram per cubic metre increase in PM2.5 concentration.

I haven’t been able to read the ACS but extraordinarily, COMEAP report no uncertainty in this 6% figure.

Perhaps it is because of this that they decide to make their own estimate. And to do this they used a procedure called elicitation.

What is elicitation?

Wikipedia defines elicitation as:

the synthesis of opinions of authorities of a subject where there is uncertainty due to insufficient data or when such data is unattainable because of physical constraints or lack of resources. Expert elicitation is essentially a scientific consensus methodology… [it] allows for … an “educated guess”.

Each expert on the committee was asked:

  • “What do you think is the chance that the correlation is not a 6% increase in mortality for a 10 microgram per cubic metre increase in PM2.5 concentration, but some other figure?

The experts were asked in turn:

  • What do you think is the chance that the true figure is greater than 0%?
  • What do you think is the chance that the true figure is greater than 1%?
  • What do you think is the chance that the true figure is greater than 2%?
  • etc. until they arrived at the figure of 17%, beyond which possible correlations were discounted as highly improbable.

The results are shown in the chart at the head of this article and in the chart at the end of the article.

And the results…

The results of this elicitation form the basis of all the mortality estimates that you have read in the previous two articles.

The results are literally, an educated guess. None of the fancy maths changes this.

The average estimate was 6% and experts considered that the true answer was ‘95% likely’ to be in the range between 0% and 15%

Since the level of PM2.5s in the UK is typically just under 10  microgram per cubic metre, it is this figure of 6% of the roughly 500,000 deaths per year that directly links to the estimate of 28,000* deaths per year.

The range of expert opinion is that the true figure could lie between 0 and 55,000: every one of the seven experts considered it marginally possible that there was no effect of pollution i.e. that the excess mortality was 0% or less.

Although the experts considered a plethora of studies – and their report is exhaustive and exhausting – in the end, this is just their opinion.

How to express their results?

COMEAP consider at length how to express the results of their elicitations. Mostly they considered that the effect on life-expectancy (a few months for most people in the UK) is clearest.

However they do specifically endorse the use of mortality to express the effect of air pollution. But they note:

… the result expressed in terms of attributable deaths or additional deaths may easily be misunderstood or misrepresented. This calculation is not [their emphasis] an estimate of the number of people whose untimely death is caused entirely by air pollution, but a way of representing the effect across the whole population when considered as a contributory factor to many more additional deaths.

...consequently we consider it unrealistic to view air pollution as the sole cause of death in a number of cases equal to the population attributable deaths.” [COMEAP 2010 Page 3]

Regarding the actual estimates COMEAP explicitly recommend that the excess mortality estimates should always include the full uncertainty estimate – i.e. including the possibility that the excess mortality may be zero.[COMEAP 2009 Page 3 Point xiii]

And my conclusion…

I agree with COMEAP that the expression of the effects of air pollution as excess deaths per year can be “easily misunderstood”. In fact I think it is nearly universally misunderstood.

And given that their recommendation that the full limits of uncertainty – which include the possibility of no effect – are generally not quoted – I think this gives quite the wrong impression.

The possibility that air pollution – mainly from traffic – might be killing 28,000* people a year – 17 times more than are killed in car accidents – is horrific.

But in fact it appears that air pollution of itself kills nobody.

Rather, when we are near the end of lives – which are now longer than they have ever been in human history – our lungs do not function as well as they might otherwise have done.

As a result, our demise – from whatever cause – is hastened.

Somehow this reality is not quite as scary as COMEAPs vision.

Other Figure

The graph as the top of the page was compiled from the responses of 7 experts. These graphs show the cumulative probabilities ascribed to a particular sensitivity coefficient by the each of the 7 experts.

The graph as the top of the page was compiled from the responses of 7 experts ‘A to G’. These graphs show the cumulative probabilities ascribed to a particular sensitivity coefficient by each of the 7 experts. Looking at any particular line, the graph tells you, in the opinion of that expert, the likelihood that the coefficient exceeds any particular % value. (SOURCE COMEAP 2009 Page 159)

Footnote

* COMEAP 2010: Page 5: Point 18 estimate 29,000 deaths per year, but this is sometimes reported as 28,000. Given the large uncertainty in the figure, I have taken the lower estimate throughout.

 

 

Did 28,000 people die from air pollution in 2012? Part 2

May 27, 2015

 

Complicated graphic showing the correlation between 'excess mortality' and a pollution event in New York in 1962. The excess mortality is shaded in red and reduced mortality in following days is shaded in blue. Click for larger image - see text for details.

Complicated graphic showing the correlation between ‘excess mortality’ and a pollution event in New York in 1962. The excess mortality is shaded in red and reduced mortality in following days is shaded in blue. Click for larger image – see text for details.

 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.

Combined Effects

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.

 

 

 

 

 

Did 28,000 people die from air pollution in 2012?

May 26, 2015
Infographic showing total deaths registered in England and Wales 2012

Infographic showing total deaths registered in England and Wales 2012. Copied from the Office for National Statistics (available here). Click image for a much larger version. Interestingly, nobody seems to die of ‘old age’!

I have been trying to work out whether – as the BBC claim – roughly 28,000 people each year die as a result of ‘air pollution’.

These reports have been going around for some time.

In 2012 a BBC headline stated that  “Traffic Pollution kills 5,000 per year” but the article text reported an “… estimate that combustion exhausts across the UK cause nearly 5,000 premature deaths each year“. There is a difference in these two claims, but the article deliberately blurs the distinction.

The same article stated that these estimates were from a wider analysis estimating “19,000” deaths in the UK from air pollution. The author contrasted this with a UK study which “found that air pollution in 2008 was responsible for about 29,000 deaths in the UK.”

The BBC are saying that air pollution is killing us. But is it?

It’s complicated, but the short answer is ‘No’.

The idea that 6% of all deaths are caused by air pollution is just wrong.

I downloaded the data for 2012 on the leading causes of death amongst men and women from the Office for National Statistics (available here).

The tables of the top 10 causes of death are listed at the end of this document and show that the only obviously breathing-related cause was Emphysema/bronchitis which was the fifth largest killer of women (5.5%) and the third largest killer of men (6%). In total this amounted to 28,533 deaths in 2012.

However only a small fraction of these deaths – perhaps 10%? – could conceivably be ascribed to air pollution. In fact the condition is commonly referred to as ‘smoker’s lung‘:

Smoking is the most important cause. Other things that make it worse are air pollution and allergy

It is also worth noting the histogram at the bottom of the chart. The only place that that one could even begin to ‘fit in’ that many extra deaths is in the categories of people aged 50 and above.

So the basic data are clear: It is untrue that 6% of all deaths are caused by air pollution in otherwise healthy people.

So how are these numbers arrived at?

A careful reading of news stories with these attention-grabbing numbers often finds that the deaths were linked to air pollution. What does that mean?

 Click for PART 2

Leading Causes of death amongst women in 2012

Leading cause of death No. of women Percentage of women
1 Dementia and Alzheimer’s 29873 11.50%
2 Heart disease 26741 10.30%
3 Stroke 21730 8.40%
4 Flu/pneumonia 15075 5.80%
5 Emphysema/bronchitis 14155 5.50%
6 Lung cancer 13575 5.20%
7 Breast cancer 10311 4%
8 Bowel cancer 6600 2.50%
9 Urinary disease 5570 2.10%
10 Heart failure 5065 2%

Leading Causes of death amongst men in 2012

Leading cause of death No. of men Percentage of men
1 Heart disease 37423 15.60%
2 Lung cancer 16698 7%
3 Emphysema/bronchitis 14378 6%
4 Stroke 14116 5.90%
5 Dementia and Alzheimer’s 13984 5.80%
6 Flu/pneumonia 11063 4.60%
7 Prostate cancer 9698 4%
8 Bowel cancer 7841 3.30%
9 Lymphoid cancer 6301 2.60%
10 Throat cancer 4603 1.90%

The strange truth about El Nino

May 13, 2015
Illustration of changes in the height of the sea surface during an El Nino Event. It's hard to know exatly what to make of this graphic, but it does look nice. Courtesy of the BBC

Satellite data showing changes in the height of the sea surface. The large straight red band just to the left (West) of South America represents a region of warmer water which has expanded and caused an increase in the height of the sea surface. This picture was stolen from the BBC web site.

You will probably hearing a lot about El Niño this year because the Australian Bureau of Meteorology have predicted that El Niño conditions will build through the coming year.

The news stories will all look like something like this:

  • Yada Yada Yada
  • Drought/Flood somewhere because of El Niño and Climate Change.
  • Isn’t it terrible

There will be nothing you can do except to experience a sense of vulnerability. And if you are of a similar disposition to me, you may also experience an increased sense of general anxiety.

However the amazing fact, which I have never seen mentioned in all my years of reading about this stuff is that, collectively, we have no idea what causes El Niño  events.

And we certainly can’t predict the events: the current ‘prediction’ is only being made because the El Niño has already begun!

So what do we know?

An Australian Bureau of Meteorology graphic describing the weather patterns in the Pacific Ocean as being either neutral, La Nina or El Nino

An Australian Bureau of Meteorology graphic describing the weather patterns in the Pacific Ocean as being either neutral, La Nina or El Nino

The term El Niño describes a set of linked atmospheric and oceanic conditions. And we understand that the weather patterns in the Pacific Ocean oscillate between three states.

  • Neutral (about 50% of the time)
  • La Niña
  • El Niño (Every 4 to 7 years)

These patterns are so large that they affect the weather right around the globe, and the oscillation between these ‘phases’ is called the El Niño Southern Oscillation (ENSO).

The Australian Bureau of Meteorology have an excellent video description of the phenomenon and its consequences for Australia.

And what don’t we know?

We don’t know what causes the oscillation from one phase of ENSO to another. And so we can’t predict changes from one phase to another.

And importantly, although it is quite conceivable that future Climate Change could affect the transitions from one phase of ENSO to another, we have no idea whether there will be any effect.

And why don’t we know it?

Well obviously, I don’t know the answer to this question. But I think it is this.

ENSO is a linked oceanic and atmospheric phenomenon.

Each of the three phases is self-sustaining i.e. changes in the wind patterns reinforce changes in the location of warm water. And changes in the location of the warmer water reinforce the changes in the wind patterns.

But the variability of the weather is such that it can move the weather patterns from one self-reinforcing phase to another.

And so these planetary scale weather events are triggered by some as yet unknown ‘local’ or ‘short-term’ variability in weather.

Things may improve. As I wrote in my review of Climate Models in the IPCC’s 5th Assessment Report, some models now spontaneously predict ENSO-like behaviour that was not programmed into the model.

And as models of the atmosphere and ocean improve, they will become better able to simulate weather on both the small scale and on the largest scales.

So as our understanding develops it seems likely that changes of ENSO phase will eventually become predictable.


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