Pollutant-specific recommendations for revised bands

This chapter presents our detailed recommendations for revised air pollution bands5 _for

particulate matter, sulphur dioxide, nitrogen dioxide and ozone. As explained above, our starting points for these recommendations were the thorough reviews of the evidence of the health effects of air pollutants by WHO (2006), although we also took other evaluations, and additional data and considerations, into account.

4.1.1 Proposed bandings for particulate matter, PM10 and PM2.5 Derivation of WHO short-term guidelines and interim targets

Studies of the effects of particulate matter (PM) on health have used a variety of different metrics including PM10, PM2.5, black smoke (BS) and total suspended particulates (TSP). Much

of the epidemiological evidence on the effects of particulate matter comes from studies in which PM10 was measured. However, the key studies of the effects of long-term exposure to

particulates report concentrations in terms of PM2.5 and these were used by WHO as the basis

for the derivation of air quality guidelines (AQG).

The long-term (annual average) guideline of 10 µg m–3 _{PM2.5 was set at a concentration}

considered to be below the most likely effects levels. Data from the American Cancer Society (ACS) cohort study (Pope et al, 2002) and the Harvard Six Cities study (Laden et al, 2006) were important in this derivation. The guideline represents the lower end of the range at which significant (more than 95% confidence) effects on survival (total, cardiopulmonary and lung cancer mortality) were observed in the ACS study. It was supported by data from the Six Cities study suggesting that effects were likely in the range 11–15 µg m–3_{. Nonetheless, WHO noted}

that there is little evidence for a threshold of effect and that adverse effects on health cannot be ruled out even below this level.

Annual average interim targets were derived based on concentrations at which effects were seen in studies of long-term exposure. The increased risk of mortality at these concentrations, compared with the risks at the guideline concentration, was calculated. The mortality risk at the interim target 3 (IT-3) level (15 µg m–3 _{PM2.5) was estimated as being 3% higher than at the}

guideline value. Guidelines and interim targets for PM10 were derived using the relationship

between PM2.5 and PM10 (approximated as a ratio of 0.5, typical of urban areas in developing

countries) to give an annual average guideline of 20 µg m–3 _{PM10 and an IT-3 of 30 µg m}–3 _PM10.

The primary goal underpinning the derivation of the WHO short-term guidelines was the control of long-term averages to ensure that the annual average guidelines would be met. The relationship between annual and 24-hour average concentrations of particulate matter was used to extrapolate the annual guidelines to short-term (24-hour mean) guidelines of 50 µg m–3 _PM10

and 25 µg m–3 _{PM2.5. WHO noted that, where these short-term guidelines are met, peaks of}

pollution that would lead to substantial excess illness or deaths are unlikely to occur. As for the short-term guideline, the short-term IT-3 (75 µg m–3 _{PM10, 37.5 µg m}–3 _{PM2.5 as a}

24-hour mean) was primarily designed to manage long-term average concentrations and was derived from the long-term IT-3 value. An effects coefficient of 0.5% per 10 µg m–3 _PM10

(based on relative risks seen in multi-city time-series studies in the USA and Europe, and meta- analyses6 _{of studies from cities elsewhere) was used to calculate the increased risk of short-term}

mortality associated with the 24-hour mean IT values, compared with the risk at the guideline. The increased short-term mortality risk at the IT-3 level was estimated to be about 1.2% (WHO, 2006). The interim target 1 (IT-1) of 150 µg m–3 _{PM10 as a 24-hour mean was set at a}

concentration estimated to cause about a 5% increase in short-term mortality compared with the AQG. The IT-2 (100 µg m–3 _{PM10) represents an approximately 2.5% increased risk. The}

24-hour average IT-1 (75 µg m–3_{) and IT-2 (50 µg m}–3_{) for PM2.5 were derived using the}

PM2.5 : PM10 relationship of 0.5.

WHO air quality guidelines and interim targets for particulate matter: 24-hour mean (taken from WHO, 2006)

24-hour mean (µg m–3_)*

Basis for the selected level PM10 PM2.5

WHO IT-1 150 75 Published risk coefficients from multicentre studies and meta-analyses (about 5% increase in short-term mortality over the WHO AQG)

WHO IT-2 100 50 Published risk coefficients from multicentre studies and meta-analyses (about 2.5% increase in short-term mortality over the AQG)

WHO IT-3 † _{75 37.5 About 1.2% increase in short-term mortality over the} AQG

WHO AQG 50 25 Relationship between 24-hour and annual particulate matter levels

* 99th _{percentile (three days per year)}

† For management purposes, based on annual average guideline values, the precise number to be determined on the basis of local frequency distribution of daily means

Recommendations for UK pollution bandings

We regard the effects of particulate matter following short-term exposures to be the most relevant to the proposed use of the air pollution banding system and, therefore, consider the effects of PM10 examined in daily time-series studies as the most appropriate

evidence base for the bands. We note that the 24-hour mean AQG recommended by WHO for PM10 (50 µg m–3) is the same as the Air Quality Standard recommended by the predecessor of

the COMEAP Standards Advisory Subgroup, the Expert Panel on Air Quality Standards (EPAQS, 1995a)7_{. EPAQS considered that this was a concentration at which effects on}

individuals following short-term exposures are likely to be small (a rise from 20 µg m–3 _{– an}

approximate annual average in the UK – to 50 µg m–3 _{was calculated as likely to result in}

one extra patient per day in a population of one million being admitted to hospital with respiratory effects).

We therefore recommend 50 µg m–3 _{as the boundary between the Low and Moderate air}

pollution bands. We propose 75 µg m–3 _{and 100 µg m}–3 _{(the same as the WHO 24-hour mean}

IT-3 and IT-2 levels) as the boundaries between the Moderate and High, and High and Very High, bands. While these boundaries are, to a certain extent, arbitrary, using current estimates, they are approximately equivalent to 1.25% and 2.5% increases, respectively, in short-term mortality compared with 50 µg m–3_.

WHO noted that, although it is believed that much of the health impact of particulate matter is due to fine particulates (PM2.5), coarse particulates (PM2.5-10) cannot be considered as

harmless and that this indicates a need for guidelines and targets for both PM2.5 and PM10. For

the same reason, we consider that the AQI should include both PM2.5 and PM10. Like WHO,

we recognise that it is possible to use the proportion of PM10 that is typically present as PM2.5

(i.e. the ratio of PM2.5 : PM10) to extrapolate guidelines/targets from one particulate matter

metric to the other. However, the mean PM2.5 : PM10 ratio in the UK is close to 0.7 8, which is

somewhat different from the 0.5 ‘global’ figure used by WHO. As WHO suggested that it may be appropriate to change the ratio used based on local data, breakpoints between bands of 35 µg m–3_{, 52.5 µg m}–3 _{(rounded to 53 µg m}–3_{) and 70 µg m}–3 _{PM2.5 are recommended, derived}

from the PM10 values using a factor of 0.7.

In considering the suitability of a ratio of 0.7 for this extrapolation in the context of an AQI reflecting risk during short-term elevations, we examined data from episodes of elevated particulate matter in London. The PM2.5 : PM10 ratio varied considerably, depending upon

whether the episode was primarily due to PM2.5 (episodes driven either by long-range transport

or by poor dispersion of locally generated particulate matter) or PM10 (coarse particle episodes).

These data confirmed the need for a PM10 index to ensure an appropriate indication of the

level of risk during coarse particle episodes. They also indicated that the derived PM2.5

breakpoints would be precautionary during episodes driven by PM2.5.

Recommended bandings for particulate matter: 24-hour mean

Pollutant Averaging period Units Low Moderate High Very High PM10 24-hour mean µg m–3 0–50 51–75 76–100 101 or more