Relative risk: Letters regarding risk and the Beauly–Denny power lines

Correspondence between me and Gemma from the Energy Department in the Scottish Government in connection with proposed pylons between Beauly and Denny. Previous posts on this are here, here and here. Gemma advises going to the Health Protection Agency, which advises her Department. 

From Gemma to me (January 2011):

You continue to express your particular concern about the relationship between exposure to electric and magnetic fields (EMFs) from power lines and an increased risk of childhood leukaemia.  In particular you state that the 'relative risk of childhood leukaemia in areas close to high voltage power lines can be up to 4', and you compare this number to the 'relative risks of lung cancer and heart disease resulting from exposure to secondary smoke commonly given as 1.2-1.3'.  Your assertion therefore is that to public health action taken by government in respect of environmental tobacco smoke is inconsistent with an approach that does not require undergrounding of power lines. 

Regarding the possibility of an increased risk of childhood leukaemia, in 2001 a Working Group of the World Health Organization's International Agency for Research on Cancer (IARC) reviewed health effects of static and Extremely Low Frequency EMFs (IARC, 2002).  They concluded that the pooled analysis of data from a number of well conducted studies (Ahlbom, 2000) had shown a fairly consistent statistical association between a doubling of the risk of childhood leukaemia and prolonged exposure to power frequency, residential ELF magnetic field strengths above 0.4 microtesla (T).  However, unlike for smoke, lung cancer and heart disease, there is no known biological mechanism for this association which has not yet been demonstrated to be causal. 

Based on this finding, the UK Health Protection Agency have estimated that, given the distribution of magnetic field exposures for the UK population and assuming that the association between magnetic fields and the risk of cancer was causal, of about 500 new cases of childhood leukaemia occurring in the United Kingdom each year, 2 cases might result from exposure to magnetic fields at exposures above 0.4 microtesla at home. 

In 2004 the Health Protection Agency reviewed their advice and introduced a note of precaution because of the persistence of the finding of an association between power frequency electric and magnetic field exposure and childhood leukaemia:

“The government should consider the need for further precautionary measures in respect of exposure of people to EMFs. In doing so it should note that the overall evidence for adverse effects of EMFs on health at levels of exposure normally experienced by the general public is weak. The least weak evidence is for the exposure of children to power frequency magnetic fields and childhood leukaemia.” 

A subsequent study by Draper et al (2005) on 'Childhood cancer in relation to distance from high voltage power lines in England and Wales: published in the British Journal of Medicine, considered the risk of developing childhood cancer in relation to residential proximity to the power lines rather than electric or magnetic field strengths.  The study involved 29,081 cases of cancer in children aged 0-14 years who were diagnosed during 1962-95. For each case, a matched control was selected. For each case and control, the distance of residence at the time of birth to the nearest high-voltage overhead power line was calculated.  The study concerned all 275 and 400 kV overhead lines in the UK and a small fraction of 132 kV lines.  Draper et al found that, compared with children born more than 600 m from a line, the risk of leukaemia was greater by a factor of 1.69 for those born within 200 m and by a factor of 1.23 for those born between 200 and 600 m; both these elevations were statistically significant*. 

However, with regard to this apparent increase in the relative risk, Draper et al reported that the increased risk seems to extend to at least 200 m, beyond the distance at which there would be elevated magnetic fields from the local high voltage power line. They commented: 

“Thus our results do not seem to be compatible with the existing data on the relation between magnetic fields and risk”.

This would suggest, therefore, that at least some of the increased leukaemia risk may be associated with factors other than EMFs, and the authors did not regard their findings as establishing a definite causal association between childhood leukaemia and exposures to EMFs from power lines.

From their results, Draper et al estimated that, of the 400-420 cases of childhood leukaemia that occur annually in England and Wales, up to five cases might be associated with living in proximity to high voltage power lines (though not necessarily caused by the electric and magnetic fields). 

This evidence, and other new studies, have not caused the Health Protection Agency to change its advice that the epidemiological evidence is not strong enough to justify the firm conclusion that electric and magnetic fields from power lines cause leukaemia in children. 

In contrast to this weak evidence of a risk and small number of potentially attributable cases, the evidence for an association between 'passive' smoking and ill health is strong and convincing, and the estimated burden of associated disease is high.  For example, Cancer Research UK note that

·        Non-smoking spouses of people who smoke at home have 27% higher risks of lung cancer.·        People exposed to second-hand smoke in the workplace have 24% higher risks of lung cancer.·        One study estimates that passive smoking may kill over 11,000 people every year in the UK from cancer, heart disease, strokes and other diseases.·        A study by the Royal College of Physicians showed that about 17,000 children in the UK are admitted to hospital every year because of illnesses caused by second-hand smoke.·        children in households where both parents smoke have a 72% higher risk of respiratory diseases.·        Childhood exposure to second-hand smoke had also been linked to a wide range of other conditions including asthma, Sudden Infant Death Syndrome (or cot death), childhood meningitis and mental disabilities.

I hope that this comparison demonstrates that the approach taken by the Scottish Government Health Directorates to the protection of public health is consistent with the best available scientific evidence and authoritative advice.

 References

AGNIR (2001). ELF electromagnetic fields and the risk of cancer. Report of an Advisory Group on Non-ionising Radiation. Doc NRPB, 12(1), 3-179.
Ahlbom A, Day N, Feychting M, Roman E et al (2000). A pooled analysis of magnetic fields and childhood leukaemia. Br J Cancer, 83(5), 692-8.
Advice on Limiting Exposure to Electromagnetic Fields (0–300 GHz). Documents of the NRPB: Volume 15, No. 2 2004
Draper GJ, Vincent T, Kroll ME and Swanson J (2005). Childhood cancer in relation to distance from high-voltage power lines in England and Wales: a case-control study. BMJ, 330(7503), 1290.

*A statistically significant association is denoted by a confidence interval that does not span unity. For example a relative risk estimated to be 1.02 with a confidence interval of 1.01–1.04 would be considered statistically significant.  A relative risk estimate of 4.02 with a confidence interval of 0.78–34.2 would not be considered significant because of the much wider uncertainly of the estimate.

 From me to Gemma (1 September 2011):

Thank you for your email from January below. I apologise for the delay in replying. You have shown me that the relative risk of leukaemia from living within 200m of a pylon is 1.69 but the risk of lung cancer from exposure to secondary smoke (from CRUK's work) is 1.27 or 1.24. Then you state (using CRUK's evidence) that the risk from secondary smoke is well established. Cancer Research primarily funds work that supports further regulation of tobacco – a bias that is surely fundamentally unscientific. You consider the possibility of confounding factors for leukaemia near electricity pylons, but not for exposure from tobacco smoke, where the statistical probability of lung cancer is lower and the likelihood of confounding factors higher. 

You define statistical significance as an elevated risk with a confidence interval that doesn't span zero, a definition that I don't find satisfactory. How can a risk factor of 1.1 clearly show causation when the elevation of risk is so small? You also bring no evidence to bear that the confidence intervals with lung cancer incidence don't straddle zero (here's one that does straddle zero).

It seems to me that there is no evidence to suggest that there is greater risk of lung cancer to adults from secondary smoke than there is of leukaemia to children from pylons. In addition the emphasis should surely be on protecting younger lives from illness at home, when adults are not permitted to take risks with their own lives at work and in recreation by exposing themselves to secondary smoke, voluntarily or otherwise.

 From Gemma to me (14 September):

Thank you for your email of 1 September 2011. I note your concerns however our position has not changed since our last correspondence with you earlier in January. I work in the Energy Consents and as you will know  we have taken our advice from the Health Protection Agency for Scotland and the UK Health Protection Agency. Last January I contacted both of these organisations personally for an up to date position citing the reports you mentioned and when they replied I included this in my previous response to you. As this is out-with our area of expertise, I can only suggest you contact the Health Protection Agency direct if you have continuing concerns. Here is a link to their website: http://www.hpa.org.uk/HPAwebHome/ 

I hope this is helpful.

I think I'll take her advice.

Beauly–Denny pylons: writing to the Minister

Following October's story on this, I wrote to the Minister for Enterprise, Energy and Tourism for clarification on the relative risk issue.

In your capacity as Energy Minister I am interested in your views on how a power line can be authorised and not undergrounded in a heavily populated area, even though the relative risk of childhood leukaemia in areas close to high voltage power lines can be up to 4 (compared with relative risks of lung cancer and heart disease resulting from exposure to secondary smoke commonly given as 1.2–1.3, yet is held to be dangerous enough to justify a comprehensive smoking ban).

In the Minister's reply (written by a Government officer) I was told that Ministers don't have the authority to order a power line to be laid underground. Their powers are limited to overhead lines.

Regarding the risks to residents of an overhead pylon, the Government has taken advice from the Health Protection Agency (a UK body), and 'their view accords with that of the World Health Organisation [no less!] – that despite extensive research, there is no evidence to conclude that exposure to low level Electro Magnetic Fields (EMF) is harmful to human health'. [emphasis added]

Well, that's very interesting, as the relative risk of childhood leukaemia from exposure to power lines is over three times higher than the risk of lung cancer from secondary smoke. 'Extensive research' in this case shows this to be a risk so unacceptable as to leave everyone terrified of secondary smoke exposure.

The letter didn't comment on secondary smoke exposure or the smoking ban. I've written again tonight:

Thank you for the reply to my email (reference xxxx) from Gemma Gallacher. 

Her final paragraph stated:

'Naturally the health of the Scottish people is of major importance to Scottish Ministers and consent would not have been granted to the application were they not utterly confident that residents along the route of the line will not be threatened.'

She did not contest the basis of my letter, that relative risks of leukaemia in persons exposed to high voltage lines rise up to four times the risk to non-exposed persons, while the risk of lung cancer to persons exposed to secondary smoke run at around 1.2 times the risk to non-exposed persons.

According to Gemma's letter the government is 'utterly' confident' that the risk to persons exposed to leukaemia (generally children) is negligible to non-existent – while the risk to those exposed to secondary smoke, although much smaller, is considered so unacceptable as to merit a smoking ban in adult venues. I've asked the Minister to comment, but it may be a month or so before he does so.

Maybe I should ask the World Health Organisation too. How do they come to the view that a relative risk of 4 is not dangerous, when a lower relative risk is responsible for this carnage?

The Beauly–Denny pylons: more about relative risk

h/tip to Kate

Scottish Government plans to build a power line between Beauly (Cromarty) and Denny (Stirlingshire) have given rise to protest on health grounds. The line will carry 400 kilo volts and cross 137 miles at a height of 53 metres. Parts of an existing line will be dismantled and parts will go underground, but no underground work is expected at Stirling, the major centre of population on the route.

A Daily Record report earlier this year mentions leukaemia as a known risk of living under electricity pylons.  It claims that childhood leukaemia rates under high voltage lines are twice what they are away from such lines, and also mentions depression and suicide as further risks associated with high voltage lines. Dr Richard Simpson MSP criticises the absence of any proper discussion of these health issues.

A doubling of risk is fairly low. To compare with the risks from secondary smoke: the risks (of lung cancer or heart attack) commonly said to result from exposure to secondary smoke are around 24 per cent, or a relative risk of 1.24, which is considerably less than 2 (indicated by a doubling of risk). Many risks have to be at least 3 before they are taken seriously enough to merit investigation as to whether they can constitute a cause of an outcome.

Dr Richard Simpson does himself some credit in this instance. He is clearly a man who believes in the precautionary principle: he favours smoking restrictions on health grounds – even though a risk of 1.24 does not constitute a meaningful risk in any sense of the word. A doubling of risk (an increase by a factor of 2) is perhaps another matter, especially when discussing childhood cancers, rather than the alleged ill effects of secondary smoke, which are generally diseases of old age. As Dr Simpson says: we may decide after an investigation about the likely health impact that we still want to have the project, but we are not even having the discussion about health. Stirling residents see the government burying the power lines in areas of scenic significance but not in areas where large populations live.

This link discusses some of the issues around electro magnetic fields (EMFs) that surround pylons. Paragraph 12 helpfully gives the exposure risk rates and compares pylons with both active and passive smoking. It says that secondary smoke exposure increases risk of heart attack from 10 in 100,000 non-smokers to 12 in 100,000 exposed non-smokers. Leukaemia rates increase from 3 in 100,000 to about 11 in 100,000 following exposure to EMFs. This is much more than double but the rate varies according to the strength of exposure, generally with results between 2 and 4.

Officially the verdict goes like this: 'the evidence does not establish that exposure to EMFs is a cause of cancer although it does provide some evidence to suggest the possibility exists'. That's probably fair, for a relative risk reaching 4. So the Scottish Government should recognise the fears of the Stirling residents and acknowledge that there is at least some ground for their fears, rather than ignoring the issue altogether.

As for the Scottish Government's view of secondary smoke, if it can make light of a relative risk of up to 4 on childhood leukaemia so far as to expose whole communities to such a risk, it is hard to understand how a risk of 1.24 (alleged risk from secondary smoke) is so serious that it can deprive every pub, club, bingo club, bowling alley (ad infinitum) in the land of any discretion in how this problem should be managed.