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Introduction

1. In the COMEAP quantification report (Department of Health, 1998), two options are given for the size of the effect of ozone on health in the UK as in the table below.

Table 1 Numbers of deaths and hospital admissions for respiratory diseases affected per year by ozone in both urban and rural areas of Great Britain during summer only (based on summer 1995)

2. This is clearly an area of uncertainty. Reducing this uncertainty, if possible, is crucial for the interpretation of the health implications of the current trends in ozone concentrations. Peak (hourly mean) ozone concentrations are falling but annual mean concentrations are not and may even be rising. If there is no threshold for ozone, this could represent a worsening rather than an improvement of the situation.

3. An updated COMEAP opinion would therefore be valuable. This paper invites the Committee’s views on the best approach to this issue. This will be used as guidance for the preparation of further papers. An opinion by the end of 2002 would allow COMEAP’s view to be fed into the EU review process.

Ozone concentrations

4. The 4^th Photochemical Oxidants Review Group report (Department for the Environment, Transport and the Regions, 1997) assessed data on ozone concentrations up to 1995/1996. It concluded that annual mean ozone concentrations were increasing slightly or decreasing slightly depending on sites (decreases at urban sites). Peak (hourly mean) concentrations were coming down. The latter was interpreted as leading to a reduction in health effects. However, this might not be true, if there is no threshold (see below). The EPAQS standard of 50ppb (8 hour average) was widely exceeded.

5. The technical annexe to the latest version of the air quality strategy (data up to 1998) suggests no obvious downward trend in annual mean or 98^th percentile (hourly mean) concentrations except possibly a small downward trend in the 98^th percentile (hourly mean) in central London (DETR, 2000). The strategy suggests that exceedances of 90ppb should be eliminated by 2010 but that emissions of ozone precursors need to be reduced by 50-75% more than the EC targets before the 1997 strategy objective could be met.

6. The sustainable development indicator (average number of days per site above air quality standards) (data up to 2000) also shows no clear trend for ozone and air quality standard exceedances due to ozone now exceed those due to particles in urban areas (as a result of decreases in particle concentrations) (Annex 1).

Approaches to assessing the health evidence

7. The chapter on ozone from the quantification report is attached at Annex 2. (This is from the COMEAP report published in 1998 and is available on request from the Secretariat). This includes at Figure 6.1 a ‘bubble plot’ suggesting a threshold between 40 and 60ppb for an effect of ozone on respiratory hospital admissions in London. However, it is also noted that this has not been seen in other studies. The report concludes that effects on both deaths brought forward and respiratory hospital admissions can be quantified. Calculations were performed both with and without a threshold of 50 ppb (see Table 1).

8. To update this view, it first needs to be confirmed that recent evidence still supports an association between ozone and deaths brought forward or respiratory hospital admissions. This can be done using the St. George’s database and is likely to be confirmed. Other endpoints (cause-specific mortality, cardiovascular admissions, respiratory symptoms and lung function from panel studies) could also be examined. Long term effects on lung function may also be important. The Committee may wish to comment on how wide a range of endpoints should be considered.

9. If associations are confirmed, consideration needs to be given to how to assess whether there is evidence of a threshold within the data. This is a complicated matter. The Committee has previously discussed the difficulties in finding a threshold at the population level given that the time series studies examine the overall impact of a range of personal exposures and a range of individual susceptibility. Some work has been done using statistical techniques to examine the shape of the dose-response function but this has not usually been conclusive – the variable data may be compatible with both a linear or a curved relationship. It is also unclear how information on the shape of a dose-response function in individual studies could best be combined across studies. Does the Committee consider it is feasible to address these issues and, if so, what is the best way to approach this?

10. In addition to the data within individual studies, it would be possible to stratify the studies in ways that could shed light on the possible existence of a threshold. For example, the time-series studies could be compared according to the range of ozone concentrations in the places where they were performed. If there were a threshold, it would be expected that it would be more difficult to detect a significant effect in places with a low range of ozone concentrations. Similarly, comparisons of effects in summer vs winter may also provide clues to threshold effects. Urban/rural comparisons (if available) are another possibility. The St. George’s database could be used to assist with generating these comparisons. Does the Committee consider these comparisons would be useful? Are there any other useful comparisons which could be suggested?

11. Chamber studies, animal studies and mechanistic studies of various types could also be assessed for information on the plausibility of a threshold in the epidemiological studies. This could be very labour intensive. How much importance does the Committee place on investigating these aspects, given that it is the epidemiological studies that would be used for quantification? Could it be concluded that a threshold exists from this type of information alone even if apparently contradicted by the epidemiological evidence? Are there any particular aspects that should be examined?

Conclusions

12. The Committee is invited to discuss the questions highlighted above. A possible series of questions defining the scope of the Committee’s review is outlined below for comment.

Is there a threshold or non-linear dose-response for ozone and health effects?

Can a value for a threshold be defined?

Do different thresholds apply for different health effects?

Are associations found in time-series studies in winter and summer and in urban and rural areas? Do the associations vary according to the range of ozone concentrations examined?

What are the implications for quantification of the effects of ozone on health?

What research is needed to further clarify this issue?

13. It would be helpful if the Committee could indicate which aspects it would like to address first for the next meeting. The Secretariat, with contributions from Members as appropriate, can then prepare further papers.

Secretariat
October 2001

References

Department for the Environment, Transport and the Regions (1997) ‘Ozone in the United Kingdom’ Fourth Report of the Photochemical Oxidants Review Group.

Department of Health (1998). Committee on the Medical Effects of Air Pollutants. Quantification of the Effects of Air Pollution on Health in the United Kingdom. London: The Stationery Office.

DETR Statistical Release Annex 1
AQ-00 4 May 2001

Air quality headline indicator for sustainable development: 2000

• In urban areas in 2000, days when air pollution was recorded as moderate or higher fell to16 days on average per site, compared with 30 days in 1999 and 23 days in 1998. The figure for 1999 was higher than in 1998 and 2000 because ozone levels were higher that year, due in part to warmer weather.
• In rural areas, air pollution was recorded as moderate or higher on 25 days on average per site in 2000 compared with 48 in 1999. The number of days has fluctuated between 21 days in 1987 and 50 days in 1990. The series can be volatile from one year to the next and there is no clear trend. This reflects the variability in levels of ozone, the main cause of pollution in rural areas.
• At urban sites, the number of days of pollution caused by particles and sulphur dioxide has continued to decline.

Chart 1: The headline indicator

The air quality headline is one of the 15 headline indicators of sustainable development. It measures the average number of days per site on which pollution levels were in ‘moderate or higher’ bands of air pollution, equivalent to levels being above National Air Quality Standards*.
*See research report Development of an indicator of overall air pollution concentrations, DETR, 1998 on http://www.environment.detr.gov.uk/airq/airpollconc/index.htm. The standards are those used in the current banding system. For ozone, in addition to the National Standard criteria for an 8 hour running mean greater than or equal to 50 ppb, a day will be in the moderate band if there is a 1 hour mean of 50-89 ppb.

Chart 2: Causes of air pollution at urban sites

The main causes of days of moderate or higher air pollution at urban sites are fine particles (PM₁₀), ozone and sulphur dioxide. Between 1993 and 2000, the average number of days of pollution at urban sites caused by fine particles, solely or in combination with other pollutants, fell by about 85 per cent to 6 days per year. The average number of polluted days caused by sulphur dioxide, solely or in combination, fell from 20 days in 1993 to less than one day in 2000. Particles come from numerous man made and natural sources, and can be generated in the UK and abroad.

The number of days caused by ozone pollution has fluctuated. Production of ozone is affected by the weather, which can also lead to ozone and the pollutants that cause it being blown over from mainland Europe. The hotter summer in 1999 led to a greater number of days of ozone pollution than in 1998 or 2000, and since 1999 ozone has caused more days of poor air quality in urban areas than particles. Ozone undergoes chemical reactions with oxides of nitrogen to form nitrogen dioxide. Urban areas tend to have higher levels of oxides of nitrogen than rural areas with the result that ozone concentrations recorded at urban sites are generally lower there than in rural ones.

When interpreting the headline indicator the following points should be borne in mind. First, in terms of public exposure, urban days of air pollution are more important than rural ones since many more people live and work in urban areas. Second, days of pollution in rural areas are concentrated in the warmer months, whereas those in urban areas are spread more evenly throughout the year. Third, the indicator only shows when the short-term health related thresholds are exceeded. For some pollutants long-term exposure is equally or more important. Emerging evidence suggests that health effects of exposure to long-term particle pollution are greater than those from exposure to short-term episodes. In some urban areas, the Air Quality Strategy targets for annual mean concentrations for particles and nitrogen dioxide (for 2004 and 2005 respectively) are currently exceeded.

Summary statistics for 2000 for individual pollutants from the automatic monitoring network were released today on the National Air Quality Information Archive at http://www.aeat.co.uk/netcen/airqual/statbase. Charts showing trends to 1999 for individual pollutants against targets are presented in the Digest of Environmental Statistics at http://www.environment.detr.gov.uk/des/index.htm and will be updated later this summer.

Table 1: Average number of days of moderate or higher air pollution per site

.. not available because of insufficient data

Note that data capture was below 75% but greater than 50% for sulphur dioxide for Manchester Piccadilly in 1998 and for Port Talbot in 1999; for nitrogen dioxide for Glasgow Centre in 2000; and for ozone and nitrogen dioxide for Narberth in 2000, The guidelines recommended by the research report¹ are that for the indicator from 1998 onwards the data capture criteria for inclusion of site data should be raised from 50% to 75%. However, DETR believe that greater consistency in trends is achieved by including the data for the above sites than by excluding them.

Notes to Editors

1. The Government’s headline indicators of sustainable development are a ‘quality of life barometer’ measuring everyday concerns like housing development, health, jobs, water quality, educational achievement, wildlife and economic prosperity. They are intended to focus public attention on what sustainable development means, and to give a broad overview of whether we are achieving a ‘better quality of life for everyone, now and for generations to come’.
   
   
2. The headline indicators together with over 130 other indicators of sustainable development were published in Quality of life counts* in December 1999. This provides a baseline assessment for monitoring and reporting on future progress towards economic, social and environmental sustainable development.
   *Quality of life counts: Indicators for a strategy for sustainable development for the United Kingdom. DETR, 1999, London (ISBN 1 85112 3431) http://www.sustainable-development.gov.uk/indicators/headline/index.htm
   Data from the monitoring networks can be found on the Department's National Air Quality Information Archive site at http://www.aeat.co.uk/netcen/airqual/statbase
   
   
3. The air quality headline indicator measures the average number of days per site on which pollution levels were above National Air Quality Standards¹. The Standards represent defined levels which avoid significant risks to health. As levels increase above the Standard, the likelihood of effects on health increases. For example, levels of ozone in the ‘high’ band may cause coughing and discomfort on deep breathing during exercise in some people.
   
   
4. The pollutants included within the indicator are particles (PM₁₀), ozone, sulphur dioxide, nitrogen dioxide and carbon monoxide. Monitoring data from the National Automated Monitoring Networks were used. Concentrations of the five pollutants were analysed to determine the number of days at each site on which the pollution was moderate, high or very high. This means concentrations for at least one of the pollutants exceeded the National Standard.
   
   
5. The main causes of moderate or higher air pollution at urban sites are ozone, PM₁₀ and sulphur dioxide. Days caused by particles and sulphur dioxide have fallen steeply since 1993 and ozone is now the main cause in urban areas. In rural areas, almost all days of air pollution are caused by ozone. The indicator for rural areas can be volatile from one year to the next and there is no clear trend. Production of ozone is affected by the weather, which can also lead to ozone and the pollutants that cause it being blown over from mainland Europe.

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