1. Health – A Precautionary Approach
23. There is scientific consensus that exposure to air pollution is harmful to people's health in terms of premature mortality and morbidity, mainly related to respiratory and cardiovascular disease. It is widely accepted that outdoor air pollution causes damage to human health across a wide range of conditions, from pre-birth to old age. Indeed, the evidence of effects of both short-term and long-term exposure continues to grow, with the greatest public health effects being associated with long-term exposure. Air pollution is also harmful to the environment generally, in particular to sensitive habitats and the wildlife depending on these, across Scotland, from local emission sources and more widely through dispersion and long-range transport of air pollutants. A detailed review of the evidence on human health impacts of air pollution was undertaken by the health and environment working group which supported the CAFS review, together with a comparison of the international and Scottish evidence. There is also emerging evidence showing a possible association between air pollution and both exacerbated symptoms and mortality levels attributed to COVID-19, although not all of this evidence will necessarily have taken into account possible confounding factors. This area of research will continue to evolve.
24. Human health improvements are not related solely to direct reductions in air pollution. Policies that improve air quality can potentially have multiple co-benefits for population health, for addressing inequality and for mitigating and adapting to climate change. A prime example is policy to promote active travel. Walking, wheeling and cycling increase physical activity, significantly reduce cardiovascular incidence and mortality, and have been shown to reduce all-cause mortality, even after controlling for other physical activity. Evidence shows that the physical activity benefits of active travel outweigh the harm caused by potentially more exposure to air pollution in all but the most extreme situations. However, walking, wheeling and cycling in places with noticeable poor air quality is a disincentive. Measures to reduce air pollution from road transport and to increase levels of active travel can therefore amplify benefits to public health.
25. Evidence continues to accumulate on the impacts of poor air quality, expanding our understanding of how air pollution is harmful to public health and the environment. Although many of the most important pollutants are now below accepted existing health based limits, areas of concern remain. Despite the general downward trend, high levels of nitrogen dioxide persist in some urban hotspots. Scotland is fully compliant with fine particulate matter targets at EU level, and almost so domestically, but there is currently no known threshold below which health impacts don't occur. This means that we must continue to take action on improving air quality across the country, including areas where targets are being met. In addition ammonia, which is a major contributor to particulate matter formation and also has direct environmental impacts, around 90% of which is generated by the agricultural sector, has not reduced at anything like the same rate as other pollutants, and even increased over some recent years.
26. More evidence is also available on effective interventions for reducing people's exposure, especially to transport-related pollution. Consequently, despite recent encouraging trends, there remains scope for further beneficial reductions.
27. Other issues that correlate closely with air pollution in terms of impacts on people and the environment also need to be taken into account, including noise (especially transport-generated noise) and greenhouse gas emissions that contribute to global climate change. Increased awareness of these interrelationships is needed, as is the potential to link co-beneficial mitigating actions. Given the close linkage between outdoor and indoor air pollution and the high proportion of time spent indoors, especially by urban dwellers, indoor air quality is also important.
28. Taking this evidence into account, in 2016 Scotland became the first country in Europe to adopt into domestic legislation the World Health Organization (WHO) guideline value for PM2.5 of 10µg m3 as an annual mean, meaning that local authorities are required to take action to reduce PM2.5 levels in areas where this objective is being exceeded. At the time of publication, the WHO was in the process of reviewing this guideline value.
29. The relationship between air pollution and mortality is complex, with multiple interacting factors, one of which may be air quality, typically contributing to the death of a specific individual. Using the recommended approach of the WHO and based on previous work undertaken by the UK's Committee on the Medical Effects of Air Pollution (COMEAP), in 2018 Health Protection Scotland (HPS) provided an estimate of approximately 1,700 attributable (premature) deaths in Scotland annually. It is important to note that attributable deaths are not actual recorded deaths in a particular year; the figure is a statistically derived estimate, intended to convey as faithfully as possible the amount of excess mortality caused by air pollution across the population as a whole. The figure should therefore not be interpreted as the number of individuals in any year where air pollution has made some contribution to earlier death; that number is unknown but is almost certainly larger. Although no figure has been calculated for the combined impact of PM2.5 and NO2 on attributable deaths, based on the PM2.5 estimate and taking into consideration internationally derived risk estimates, around 2,000 attributable deaths annually may be a reasonable number.
30. There is some uncertainty from international studies about the scale of health effects associated with low pollutant concentrations typical of those found in much of Scotland today. The body of Scottish research, while relatively small, has repeatedly demonstrated impacts of pollutants on respiratory illness that are consistent with international evidence. There is also growing evidence from around the world showing associations of air pollution with other important health conditions including cardiovascular disease, dementia, diabetes, and adverse pregnancy outcomes (low birth weight and prematurity). Collectively this constitutes good evidence that air pollution, even at the concentrations found in much of Scotland, is linked to excess ill health. Consequently, despite the recent encouraging trends in air pollution in Scotland to date, there remains scope for further beneficial reduction. However, given the relatively low ambient pollutant levels across most of the country, it is difficult to predict, and is likely to be hard to demonstrate accurately, the level of additional health gain that might result from further reductions in air pollution.
31. Achieving these aims will require concerted action to make health-focused policy development more of a joint priority across all relevant central and local government departments. To achieve meaningful change, everyone – government, business and industry, employers (private and public) and the general public – will need to be encouraged to play their part in helping to reduce the future health burden associated with avoidable air pollution.
32. The inequalities issues associated with the effects and impacts of air pollution on health require further consideration. The Scottish Index of Multiple Deprivation (SIMD) contains extensive and valuable data which can be used to explore evidence of links between socially deprived communities and air quality. The relationship between deprivation and air quality is complex, and it is not always the case that the most deprived areas will experience the worst air quality. At the same time, those who generate the least air pollution are often those who suffer its effects most. Low access to economic opportunity often combines with poor health, low activity levels and reduced access to affordable mobility, all of which have the potential to exacerbate the impacts of air pollution.
33. The key, therefore, when implementing measures to tackle air pollution must be to avoid inadvertently embedding environmental injustice into proposed solutions. For example when introducing Low Emission Zones and other controlled access schemes, careful planning is needed to ensure that the vehicles affected are not simply displaced into surrounding areas.
34. Air pollution causes harm to human health and, while significant improvements have been made, the impacts continue to be felt. Whilst there is increasing evidence of both population-level and specific impacts that require to be addressed, causation is often hard to prove given the multiple and interlinked factors affecting health. More action is required, both in order to achieve legal compliance with domestic and international standards and to further improve the overall health of the population of Scotland, in particular those most vulnerable members of society upon whom air pollution can have the most acute impacts. We must work coherently and effectively together, across central and local government, to develop and implement integrated health-focused policies which deliver lower air pollution and better health outcomes.
Indoor air quality
35. Urban populations in the UK spend around 90% of their time indoors; the quality of the indoor air is therefore at least as important as that of outdoor air. Indoor air quality is influenced by multiple factors including ambient outdoor air pollution. This makes estimating the health impacts of indoor air quality alone very challenging. Unlike outdoor air quality, there are no regulated limits for indoor pollutants in domestic settings in the UK. The WHO published guidelines in 2010 on safe concentrations of indoor air pollutants for general use, and the Health and Safety Executive (HSE) publishes occupational limits for a range of workplace air pollutants. Guidance on domestic indoor air quality has been produced by the National Institute for Health and Care Excellence (NICE) and, in January 2020, the Royal College of Paediatrics and Child Health (RCPCH) published a review on the impacts of indoor air quality on children and young people.
36. Given the wide range of factors contributing to indoor air quality, no single body or organisation can realistically have sole responsibility for addressing it. Thus there is a need for policy integration and coherence to avoid the risks of unintended consequences. Non-health-related developments (for example relating to building standards, furnishings or cleaning products) could have unexpected adverse health impacts if a wider perspective is not taken.
37. In-vehicle air quality studies tend to compare outside and inside air pollution levels with evidence suggesting that air quality inside vehicles can be significantly poorer when compared to typical roadside pollution levels experienced by pedestrians and cyclists. Very few studies compare different transport modes on the same routes at the same time. The measurement of in-vehicle air pollution – and the associated development of risk reduction strategies – is still an emerging science. There is also limited research into in-vehicle air quality caused by the use of the recirculation mode on vehicle air filter systems and its impacts on driver health, wellbeing and alertness.
38. In-vehicle air quality personal exposure can be influenced by a complex array of factors beyond the transport mode choice, such as traffic conditions, traffic intensity and road type. Mitigation actions to reduce in-cabin air pollution may include keeping a safe distance from vehicles ahead, keeping windows closed when in traffic (which may presumably be undertaken by the majority of drivers) and setting the vehicle ventilation for a short period of time to recirculation mode. There is also emerging evidence on the role of in-vehicle CO2 associated with using the recirculation mode in a car with multiple passengers on a non-local journey.
- Assess the evidence on health impacts of low level pollution in countries with levels of ambient air pollution comparable to Scotland.
- Commission population research on the long-term effects of air pollution using cohort methods to aid further understanding of health impacts and explain the apparently different epidemiology in Scotland.
- Convene a task group to identify what, if any, actions might best be undertaken at Scottish level to address the issues associated with indoor air pollution.
- Commission an assessment of actual exposures experienced by a representative sample of the Scottish population, assessing pollution exposures over a realistic activity range during a normal time period.
- Contribute to research on in-vehicle air quality measurement methodology, the use of recirculation mode for long-distance journeys related to CO2 and in-vehicle air pollution related to occupational health.
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