Chronic Effects of Air Pollution Begin Before Birth

Dr Robin Russell-Jones MA FRCP FRCPath

A ir Pollution is a global problem. 90% of the urban population worldwide are exposed to levels above the WHO guideline limit for small particulates (PM2.5) of 10 microgrammes per cubic metre of air (µg/m3). (1) Recent studies estimate that air pollution causes 8.8 million premature deaths globally, which represents 15.7% of all deaths. (2) This makes ambient air pollution the greatest preventable cause of death globally, bigger even than smoking at 7.2 million deaths.

The new estimate for mortality in the UK is 64,000 premature deaths annually. UK citizens lose on average 2.2 years of life due to a health hazard that is beyond their control. These latest estimates are based on 41 studies from 16 different countries . (3)

Excess mortality is only a small part of the total picture. A joint Royal Colleges report from 2016, based on 40,000 deaths annually, estimated that air pollution in the UK costs the economy £20 billion per annum, with mortality contributing only 8% of the total. (4) The rest relates to morbidity - in other words the chronic effects of air pollution which begin before birth, and persist throughout life. This review will examine some of these other effects.

Birth weight

Birth-weight is a critical measure. An analysis of 32 studies linking pregnancy outcomes with the level of small particulates (PM2.5) concluded that each increase of 10 µg/m3 in PM2.5 lowers birth-weight by 16 grams. (5) It is well known that smoking during pregnancy

also lowers birth weight, but mothers have the option of stopping, and most do. Living in urban areas does not give pregnant women this option. In Central London pregnant women are exposed to a level of small particulates that will lower the birth weight of their baby by around 24 grams. It is the equivalent of passive smoking twenty four hours a day for 9 months. (6)

Levels of PM2.5 in the capital are below the annual EU limit of 25 µg/m3, but the health effects of small particulates are without threshold. The EU commission are currently reviewing the Ambient Air Quality Directive and are likely to lower the annual limit. In the US, the legal limit is 12 µg/m3, and 8 µg/m3 in Australia.

During the Olympic Games in 2008, the Chinese Government made every effort to reduce pollution levels in Beijing, notably by restricting vehicle access, and birth weight increased. (7) The maximum benefit (23 grams) was for women who were in the last trimester of their pregnancy during the Olympic Games -in other words the stage of pregnancy associated with maximum foetal growth.

Exposure in utero

Low birth weight is linked to a host of adverse outcomes in later life, including lower IQ. Evidence also exists for a direct impact of pollution during pregnancy. Polycyclic Aromatic Hydrocarbons (PAHs) are generated by any combustion process, so members of the public are exposed from activities such as smoking, cooking, domestic fires, and of course traffic, particularly diesel. Researchers in New York have reported that exposure to PAHs during pregnancy in non-smoking mothers is linked to developmental delay at 3 years; (8) an IQ reduction of 4-5 points at 5 years; (9) increased anxiety, depression and inattention at 6-7 years; (10) reduced surface white matter in the brain at 8 years; (11) and delayed self-regulatory behaviour that was most significant at 11 years. (12)

These observations have not yet been replicated in the UK. Even so the situation may be worse as diesel vehicles represent a much larger proportion of the UK car market than in the US. Since 2000 the proportion of new vehicles which run on diesel has increased from 14% of the new car market to almost 50%. Levels of Benzo-a-pyrene, the only PAH monitored in the EU, have increased by 52% at traffic monitoring sites since 2000 . (13)

Effect on IQ

These observations have profound implications for public health and educational attainment. UK children and teenagers are currently suffering a miniepidemic of mental health prob-

lems which the medical profession are struggling to explain. Commentators have targeted social media, but the toxic impact of air pollution on developing brains is equally worthy of attention, and a vital area for further research.

Thus air pollution has also been linked with IQ deficits in teenagers and a recent Londonbased study showed higher rates of depression amongst teenagers exposed to higher levels of air pollution. (14),(15) It could be argued that these associations are not causal, but intervention studies are compelling and cannot be dismissed so easily. For instance, a recent US study demonstrated an increase in the cognitive performance of school children when air filters were fitted to schools in Los Angeles. (16)

Dementia

There are also data showing that air pollution also impacts cognitive function in later life. A systematic review in 2016 identified 31 studies linking air pollution with cognitive decline: 15 in the Americas, 5 in Asia and 11 in Europe. (17) In 2017 a study of 2.2 million older adults living in Ontario showed that their chances of developing dementia increased the closer they lived to a major highway, indicating a dose-response relationship for traffic-derived pollution and dementia. (18) In 2018 a study of 75 GP practices in Greater London showed that the chances of developing dementia was linked to ambient levels of both PM2.5 and NO2. (19) The authors concluded that 14% of dementia cases in the UK are attributable to air pollution.

Measures needed

It is obvious that the UK Government should be doing more to mitigate these health effects. HMG could increase the tax on diesel, introduce a diesel scrappage scheme, bring forward the phase-out date for fossil fuel vehicles from 2035 to 2030, and give councils the powers to identify and fine highly polluting vehicles at the roadside. (20) They should also introduce WHO limits by 2030 at the latest with a mandatory air quality standard for PM2.5 of 10 µg/m3, and an intermediate target of 12 µg/m3 by 2025.

Instead, DEFRA has produced a Clean Air Strategy which conspicuously fails to address the problem posed by diesel. (21) It contains a commitment to halve the number of people in the UK exposed to levels above the WHO limit for PM2.5 by 2025. This is not a standard. This is an aspiration that lacks any legal force.

It is for these reasons that the UK needs a Clean Air Bill and a National Clean Air Agency tasked with responsibility for both indoor and outdoor air quality.

Dr Robin Russell-Jones is Scientific Advisor to the APPG on Air Pollution. He was previously Chair of CLEAR, The Campaign for Lead Free Air.