Sarah McMahon presents the preliminary findings of a study into traffic pollution and the respiratory health of school children living in the Bristol area.

During the winter of 1999 to 2000, a study was carried out by Bristol City Council in partnership with South Gloucestershire Council, North Somerset Council, Bath and North East Somerset Council, the British Lung Foundation and Avon Health Authority, to look at the relationship between health and traffic pollution. The study aimed to examine the effects of traffic pollution on the respiratory health of asthmatic primary school children in Bristol and the surrounding environment, and consider whether local air quality management is a means to improve health.

Winter traffic pollution was monitored at each school for two terms covering September 1999 to April 2000 (fine particles and nitrogen dioxide are both known to worsen respiratory illness). At the same time, weather conditions were also monitored. Respiratory health was assessed by asking families to monitor, using a daily diary, how often ventolin inhalers were used by asthmatic children at nine primary schools within the region - four inside and five outside Bristol.

A questionnaire survey was carried out among study children and a control group at each school to assess the influence of other factors that may contribute to their respiratory health. Fifty-six children participated in the study initially, dropping to 35 by the end. This drop was due to a number of reasons, the most common being an improvement in the child's asthma early in the study so that a ventolin inhaler was no longer in use. Of the children studied, 66 per cent were male and 64 per cent were aged seven years or more.

AIR POLLUTION
Air pollution levels were monitored across the study region using continuous analysers. The highest levels of traffic pollution (nitrogen dioxide - NO2 - and fine particles - PM10) were recorded next to busy roads. On the days when levels of air pollution were highest at schools in these locations they were also elevated at schools away from heavily traffic, but to a lesser extent. Pollution measurements at semi-rural schools used in this study were able to detect the same pollution episodes experienced in urban areas. During the study the average levels of pollution were within the standards for national air quality. Study days with the highest pollution (10 per cent of total study days) did not exceed the "moderate" category for UK air pollution banding.

Levels of fine particles and nitrogen dioxide usually peaked during pollution events. However, during early February and early March levels of fine particles were sometimes high when nitrogen dioxide from traffic pollution was not, indicating sources of particles from elsewhere. During the study the wind was predominantly from the west and south west. The strongest winds were from the west and on windy days traffic pollution levels were often at their lowest due to good dispersion. There was a strong relationship between high wind speed and low pollution.

RESPIRATORY HEALTH
Thirty-seven of the study children completed questionnaires on their family history and home environment, and this number was matched with a control group. Twenty-two of the study children had a family history of asthma and most of these children lived in semi-rural locations. When comparing study and control groups it was apparent that many of the risk factors for asthma were higher for the study children and their families, particularly: family history of asthma (60 per cent); use of antibiotic in the first two years of life (81 per cent); and condensation in the home (27 per cent), see figure 1.

AIR POLLUTION AND INHALER USE
During September to December 1999, the local area experienced a flu epidemic. Respiratory illness at school and absenteeism was so high that the affect of air pollution on health was minimal. During January to March 2000, when the general health of the region's population had improved, a relationship was found between inhaler use and pollution levels. The results were split into smaller data sets to assess the significance of asthma type and location.

Linear regression was used and the correlation was tested for statistical significance at the 99.9 per cent confidence level (when the correlation coefficient is greater than 0.32, as marked on each graph). During this period it was found that:

• inhaler use by study children had a statistically significant relationship with both fine particles and NO2 concentrations;
• the health effect was most significant one to three days after pollution for children with a family history of asthma, and four to seven days after pollution for children with no asthmatics in the family (see figure 2 and figure 3);
• inhaler use by children attending school in heavily trafficked areas only showed a statistically significant relationship with NO2 up to five days after pollution (see figure 4); and
• inhaler use by children attending school in low traffic/semi-rural areas showed a statistically significant relationship with fine particles and NO2, with a health effect three to 11 days after pollution (see figure 5).
  Results indicate that moderate increases in urban and regional pollution were associated with increased "wheezing" and use of ventolin inhalers. This relationship varied with type of pollutant, family history of asthma and location.

Fine particles: PM10 had the strongest correlation with inhaler use by children with a family history of allergic asthma, and those living in semi-rural areas. The majority of children at the semi-rural schools had this type of asthma so this association is not unexpected. It also indicates how particles can affect the whole region, particularly when trans-boundary dust is recorded. No more than 30 per cent of particulates is contributed by traffic and it is quite likely that the children in this study also reacted to non-traffic particles from varied sources.

Nitrogen dioxide: NO2 is a good indicator of traffic emissions and inhaler use by children, regardless of family history, was correlated to traffic pollution in all areas. In urban areas where the relationship is strongest, high NO2 concentrations can be used as a proxy for "wheezing" during the following week. Weather, as well as traffic volume, has a role to play in pollution episodes. While emissions from traffic remain fairly constant, low wind speeds can create a lack of dispersion and a build up of pollutants at ground level.

A similar study was carried out in Bristol between 1993 and 1994. At the time, the average level of fine particles was 40 per cent higher than during this study. Yet the relationship between pollution and increased inhaler use is still present despite the improvement in ambient air quality, why?

The "hygiene hypothesis" is based on the idea that the immune system needs a "kick start" in the early years of life, and a reduction in the frequency of common childhood infections over the years may explain the recent increase in allergic disease. The use of antibiotics in early childhood may also suppress the development of the immune system. These children are then more sensitised to low levels of pollution. Medical experts see the rise in asthma as not linked to traffic pollution, although pollutants such as fine particles and NO2 can exacerbate respiratory conditions.

In view of the study findings, the relationship between traffic pollution and increased inhaler use is not dependant on living in close proximity to heavy traffic, and local air quality management in urban areas is unlikely to have a large impact on asthma. Regional pollution episodes, triggered by adverse weather conditions, may reduce if traffic emissions drop, but pollutants from other regions of the UK and Europe will continue to impact upon health.

Use of ventolin inhalers by over 50 primary school children living Bristol and the surrounding semi-rural environment was found to have a statistically significant correlation with fine particles and NO2, regardless of location. The children were responding to the same pollution episodes, often occurring when wind speeds were low, that affected the entire study region. This relationship occurred on approximately 10 per cent of study days when pollutant levels still remained within annual national air quality standards.

For further details of the study, contact Sarah McMahon MSc CIEH, environmental quality unit, sustainable development, Bristol City Council. Tel: 0117 922 3306. Fax: 0117 922 4433 or e-mail: sarah_mcmahon@bristol-city.gov.uk

REFERENCES
Kunzli,N et al "Public health impact of outdoor and traffic related air pollution: a European assessment". The Lancet, Vol.356, 2000.
Bath & North East Somerset Council, "Air Quality Report" 1997/98.
Bristol City Council "Review and assessment of air quality in Bristol, Stage 1 & 3", 1999/2000.
South Gloucestershire Council "Air quality in South Gloucestershire", 1998.
McMahon,S.K "Asthma in children", Environmental Health Journal, Vol.103/7, 1995.