A revision of the WHO’s air quality guidelines is long overdue

As our knowledge of the damage that particulate matter causes to our health, World Health Organisation (WHO) guidelines must be updated, writes Graham Harker, senior managing consultant, Ramboll.


The publication of the Clean Air Strategy 2019 earlier this year brought the issue of fine particulates (PM2.5) to the fore and out of the shadow of compliance with the annual mean EU limit value for nitrogen dioxide.

The promise to reduce PM2.5 concentrations everywhere, and halve the number of people living in locations above the WHO guideline level of 10µg/m3 by 2025 was eye-catching.

A subsequent pronouncement by Michael Gove, then Environment Secretary, that the government’s Environment Bill would include a legally binding commitment to meet the WHO guideline values for particulate matter raised expectations further.

With the recent publication of the bill, some of those hopes may have been dashed, but perhaps more importantly for public health, the WHO air quality guidelines themselves should be reviewed.

15-year-old guidelines

The current set of guidelines was published in 2005, following a global update for particulate matter, ozone and nitrogen dioxide. Given the prominence attached to complying with the current guideline values for particulate matter (PM2.5 in particular), it is important to understand what the guideline values are and how they may change in the forthcoming revision; with the first draft due by summer 2020.

Careful examination of the 2005 guidelines provides an answer to the former question, whilst we can only speculate on the latter.

The WHO guidelines aim to provide a uniform scientific basis for understanding the effects of air pollution on human health.

They are intended to act as a basis for setting air quality management standards at a local or national level. Their values are not intended to simply be adopted as a standard or limit value, but rather to act as a starting point for the development of country-specific standards.

When developing a country-specific standard, political, economic and social considerations need to be taken into account, as these influence what is considered to be an acceptable level of risk to public health in each individual country.

In setting the current guidelines for particulates, a comprehensive review of the scientific evidence regarding epidemiological and toxicological effects was undertaken. The value of 10µg/m3 for PM2.5 was chosen as it represented a concentration that effectively reduced health risks to an acceptable level worldwide, although it is not explicitly stated what an acceptable level of risk is. 10µg/m3 is not a threshold below which no adverse effects were identified.

In fact, it is explicitly stated in the 2005 guidelines that no such concentration could be detected, and that adverse effects were identified at concentrations not much greater than background concentrations (3-5µg/m3). There is therefore benefit in reducing concentrations below 10µg/m3, especially since the 2005 guidelines encouraged countries to consider setting lower standards in accordance with their individual circumstances.

A similar process to that undertaken for the 2005 guidelines will be used for the forthcoming update. However, the evidence base has expanded considerably since the 2005 guidelines were issued, and how it is interpreted will determine whether any changes are required to the guideline values.

In particular, consideration will need to be given to whether a better pollution indicator exists to describe and control the health effects of particulate matter.

Candidates for a new pollution indicator include ultra-fine particles (PM0.1), black carbon, particle number concentration, or oxidative potential.

The complex nature of particulate matter pollution makes the process of updating the guidelines difficult. If the review of the latest epidemiological evidence is sufficient to change the conclusions regarding the exposure-response relationship of the existing PM2.5 metric, or one or more of the alternative metrics, then this could lead to changes in the guidelines.

However it must also be recognised that, in order to be useful, the metric needs to be measurable and controllable to affect a change in the health outcomes of the population. This is explicitly recognised in the current guidelines, which based the numerical guideline values for PM2.5 on health studies. Meanwhile, the PM10 guideline values were set based on a ratio of twice the PM2.5 value, which is the typical ratio observed in urban areas of developing countries.

PM10 was included as a pollution indicator, as it was relevant to the majority of the epidemiological data and for which there was, and continues to be, more extensive worldwide measurement data.

Environment Bill

What, therefore, are the implications of the standard-setting process outlined in the recent Environment Bill? The bill requires the Secretary of State to set a target for annual mean PM2.5 concentrations taking into account advice from independent experts, but a target can only be set if the Secretary of State is satisfied that the target can be met.

The target will, therefore, need to factor in economic viability and practical deliverability, and it will be interesting to see how the independent health-based advice is balanced against the judgement of whether a target can be met.

Pressure will need to be maintained on the government to be ambitious in its target setting if the maximum benefits for public health are to be realised. It may be that the timetable for setting the PM2.5 target will help in this regard.

The PM2.5 target must be set by 31 October 2022, by which time the content of the updated WHO guidelines will likely be known, and therefore can be used to ensure that the most appropriate target is set.

Surprising as it may seem, the Environment Bill is consistent with the 2005 WHO guidelines as it does not simply adopt the current guideline value. Whilst requiring a specific target for annual mean PM2.5 to be set, the bill does not explicitly deal with the issue of reducing population exposure below 10µg/m3.

By 2030, if the policies in the 2019 Clean Air Strategy are successful, the majority of the UK population will be living in areas with PM2.5 concentrations lower than 10µg/m3. From a public health point of view, there will still be a benefit in reducing PM2.5 concentrations below 10µg/m3 and, therefore, an exposure reduction target will need to be included in air quality regulations if the requirements of the WHO guidelines are to be met.


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