Exposure to higher levels of particulate air pollution is associated with higher systolic blood pressure (BP) among adolescents living in London, whereas nitrogen dioxide lowers their BP, according to a new analysis led by King’s College London (KCL) and published in PLOS ONE.
Air pollutant exposure generally has been linked with a greater risk of both cardiovascular and respiratory disease, as well as hospital visits, and death. However, thus far most studies on air pollution and blood pressure have focused on adults. Yet rapid growth in childhood and adolescence "render organ systems particularly susceptible to injury", the authors said.
"Adolescents' rapidly growing bodies may be particularly susceptible to long-lasting effects of exposure to air pollutants, including effects on blood pressure."
The researchers therefore conducted an observational study using data collected as part of the Determinants of Adolescent Social Well-Being and Health (DASH) study, a longitudinal study that tracks the well-being of an ethnically diverse cohort of London schoolchildren, reflecting the diversity of London's multi-ethnic population. DASH "seeks to understand what contributes to ethnic differences in physical and mental health over the life course", and is one of few studies worldwide to include BP measurement data in childhood and adolescence.
The team used sex-stratified, linear random-effects modelling to examine how modelled residential exposure to annual average nitrogen dioxide (NO2), particulate matter (PM2.5, PM10), and ozone (O3) might affect blood pressure in teenagers. Their estimates were based on a sample of 3284 adolescents recruited from 51 schools and followed from ages 11-13 and 14-16. Four-fifths (80%) of the children were from ethnic minority groups. Exposure estimates were based on annual mean levels of pollutants where each participant lived.
Varying Results With Different Pollutants
Their results showed that greater estimated exposure to nitrogen dioxide was associated with lower systolic blood pressure, while exposure to higher levels of particulate matter 2.5 (PM2.5) – that come from car exhaust fumes, building and industry materials – was associated with higher systolic blood pressure. The model found no evidence of a relationship of either pollutant with diastolic blood pressure.
Associations between pollutant levels and systolic pressure were stronger for girls than for boys, such that each 1μg/m3 increase in nitrogen dioxide was associated with a 0.30 mmHg (95% CI 0.18 to 0.40) decrease in systolic blood pressure for girls and 0.19 mmHg (95% CI 0.07 to 0.31) decrease in systolic blood pressure for boys. Meanwhile, a 1μg/ m3 increase in PM2.5 was associated with a 1.34 mmHg (95% CI 0.85 to 1.82) increase in systolic BP for girls and a 0.57 mmHg (95% CI 0.04 to 1.03) increase in systolic BP for boys.
Although the residential estimates suggested that adolescents from ethnic minority groups were exposed to higher annual average concentrations of pollution at home than their White peers, the associations did not vary by ethnicity. Nor did they vary by body size or socioeconomic status, and they were robust to adjustments for noise levels and lung function at 11–13 years.
The team concluded that their study indicated: "Air pollution appears to have a considerable impact on BP during adolescence, and in girls more than boys." They noted that whilst they had originally hypothesised that long-term exposure to poor air quality would be associated with higher BP, in fact they found "a mixed picture, varying in direction by pollutant" and that "associations were significantly modified by sex".
Why Nitrogen Dioxide Lowers Blood Pressure
In London, nitrogen dioxide pollution is predominately due to diesel traffic, the researchers said, noting that in their study systolic BP decreased by approximately 5 mmHg for boys and 8 mmHg for girls when NO2 almost doubled from a low to a high concentration.
"Previous research has shown NO2 may have damaging effects on the respiratory system, but the impacts of the pollutant on the cardiovascular system is less clear," they said. However, a BP-lowering effect was "biologically plausible". Animal studies had suggested that inhaled NO2 may increase plasma nitrite, which is reduced in the body to nitric oxide – a chemical known to lower blood pressure in humans and protect the myocardium against ischaemia-reperfusion injury.
A recent study at KCL found that sitting next to a lit gas cooker emitting NO2 acutely lowers blood pressure in healthy adults by approximately 5 mmHg. That effect was explained by a rapid increase in circulating nitrite concentration in the blood.
Study co-author Dr Andrew Webb, a clinical senior lecturer from KCL, said: "The effect of NO2 on blood pressure is similar to what we and other researchers have observed previously after ingesting green leafy vegetables or beetroot juice. These are rich in dietary nitrate (NO3), which increases circulating nitrite (NO2-) concentration in the blood and lowers blood pressure, an effect that may also be sustained over weeks or months with continued ingestion of nitrate-rich vegetables.
"As NO2 also increases circulating nitrite (NO2-) concentration, this provides a potential explanation as to why elevated NO2 appears to be associated with lower blood pressure in the adolescents over years."
Contrast Between Nitrogen Dioxide and Particulate Pollution
The authors concluded: "Associations between air pollutants and BP vary by sex in adolescence. Further longitudinal studies are needed to clarify the contrasting associations of ambient concentrations of NO2 versus fine particulate matter and BP levels in young people from different socio-economic backgrounds. Understanding the social and biological mechanisms linking air pollution exposure to BP over the life course is [a] major research and clinical gap."
Corresponding author Dr Alexis Karamanos, a research associate at KCL, said: "The findings highlight the potential detrimental role of exposure to higher concentrations of particulate matter on adolescents' blood pressure levels."
"Further studies following the same adolescents over time in different socio-economic contexts are needed to understand whether and how exposure to higher pollutant concentrations may affect differently the cardiovascular health of children and adolescents."
Adverse Effects May Tract to Adulthood
Blood pressure tracks from childhood to adulthood, the researchers cautioned. "Adverse impacts of air pollution in early life may track from childhood, with long lasting consequences over the life course."
The researchers also noted that levels of nitrogen dioxide and PM2.5 in London remained well above World Health Organisation Guidelines for the full duration of the study, even though annual average concentrations of PM2.5 fell between 2003 and 2006. They called for further studies to help confirm and clarify these findings, particularly among young people from different socioeconomic backgrounds.
Senior author Seeromanie Harding, professor of social epidemiology at KCL, said: "This longitudinal study provides a unique opportunity to track exposures of adolescents living in deprived neighbourhoods. Given that more than 1 million under 18s live in neighbourhoods where air pollution is higher than the recommended health standards, there is an urgent need for more of these studies to gain an in-depth understanding of the threats and opportunities to young people’s development."
Dangers to Lung Health
Asked to comment on the study by Medscape News UK, Tim Dexter, clean air policy manager at Asthma + Lung UK said: "Lung cancer, respiratory disease, heart disease, dementia and premature death – how long does the list need to grow before the Government tackles this public health emergency head on? This new research is yet more evidence of the negative impacts of air pollution on our health. Air pollution, especially PM2.5, can penetrate deep into someone's respiratory system, leading to new lung conditions and for those with a lung condition it can leave them fighting for breath.
"At the moment there is a lack of public awareness and understanding about just how dangerous air pollution can be for our health and what people can do to improve the air they breathe. At Asthma + Lung UK, we're calling on the government to provide the general public with more education about the dangers of air pollution. The government must set up PM2.5 monitoring stations in every community, so local authorities can accurately identify hotspots and implement more targeted interventions, including air pollution alerts when air pollution levels are high."
'Concerning' Risk of Cardiovascular Disease
Dr Mark Miller, senior research scientist at the British Heart Foundation Centre for Cardiovascular Science at the University of Edinburgh, told Medscape News UK: "High blood pressure is a major risk factor for cardiovascular disease, and this new study associating adolescents as young as 11 years of age with higher blood pressure from air pollution is concerning.
"British Heart Foundation-funded research has already demonstrated that tiny toxic particles in air pollution can have a range of negative effects on the heart and blood vessels. While the increases in blood pressure observed in the new study may seem small, differences like this across a whole population would greatly increase the risk of cardiovascular disease. There is an urgent need to reduce air pollution and the cardiovascular effects that are associated with it."
The study was funded by the MRC North Central London Consortium and the Primary Care Research Network. This work was also supported by the MRC Centre for Environment and Health, which is currently funded by the Medical Research Council. Infrastructure support for the Department of Epidemiology and Biostatistics was provided by the NIHR Imperial Biomedical Research Centre. The authors declared no competing interests.