Very high concentrations of nitrogen dioxide can cause constriction of the bronchioles, increased sensitivity to allergens and increase deaths rates but these occur at much higher concentrations than those found in outdoor air
Nitrogen dioxide is a member of a family of compounds: the oxides of nitrogen. The simplest member of this family, nitric oxide (NO) and some nitrogen dioxide (NO2) is produced by combustion processes: atmospheric nitrogen is combined with oxygen. Nitric oxide is oxidised to nitrogen dioxide in the air: slowly by oxygen but much more rapidly by ozone. It occurs close to sources such as road traffic or gas cooking.
Adverse health effects of nitrogen dioxide are seen in human clinical and animal toxicology studies but these occur at much higher concentrations than those found in the outdoor air. These studies have shown that at very high concentrations, Nitrogen dioxide acts as an irritant causing inflammation of the airways. By affecting the immune cells in the lungs, it can also increase susceptibility to respiratory infections.
Time series epidemiological studies have shown associations between increased level of nitrogen dioxide and increased risk of mortality (deaths) and hospital admissions but it is unclear whether these associations are directly due to nitrogen dioxide or other outdoor air pollutants, such as particles.
Oxides of nitrogen and particle concentrations are highly correlated in air samples from city roadside sites. It is particularly challenging to distinguish their effects. Some of the effects attributed to nitrogen dioxide may have been caused by particles instead. These studies have so far not been able to separate out the effects of nitrogen dioxide from those of other pollutants.
In the 1998 report The quantification of the effects of air pollution on health in the United Kingdom, based on the evidence available, COMEAP did not include the effects of nitrogen dioxide in its quantification of the health effects of air pollution.
COMEAP recommended a coefficient to calculate the possible effects of short-term exposure to NO2 on respiratory hospital admissions (see Table 1) but acknowledged the uncertainties about the relationship between exposure to nitrogen dioxide and effects on health.
Respiratory hospital admissions
+2.5% per 50 µg/m3 increase in NO2 24 hour mean
Table 1 Estimates of Coefficient to quantify the health effects of short-term exposure to nitrogen dioxide (NO2) COMEAP 1998
The evidence relating long-term exposure to nitrogen dioxide and health effects is very limited. It is currently not possible to quantify the direct effects of nitrogen dioxide on respiratory morbidity in children
COMEAP concluded that there was insufficient evidence to relate long-term exposure to nitrogen dioxide to effects on mortality (deaths) and so has not recommended a coefficient for quantifying the impact of long-term exposure to NO2 on mortality. See the 2009 report, Long-Term Exposure to Air Pollution: Effect on Mortality, PDF1.51 MB
Later in 2009 COMEAP considered whether it was possible to quantify the possible morbidity (ill health) effects of long-term exposure to nitrogen dioxide. They concluded that: it was not possible to quantify the direct effects of nitrogen dioxide on respiratory morbidity in children.
- Noted that in the studies there was a lack of control for ultra-fine particles, an important component of the traffic-related pollution mixture, which could mean that some of the reported effects of nitrogen dioxide could be confounded by exposure to ultra-fine particles.
- Agreed that there was some evidence of an association between outdoor air pollution, in general, and of traffic-related air pollution in particular, on respiratory morbidity in children.
- Members agreed that whilst, on the basis of epidemiological evidence, a direct effect of nitrogen dioxide on respiratory morbidity in children could not be clearly identified (because the possible adverse effects of nitrogen dioxide could not be disentangled from those of the other pollutants in the urban mixture), a small effect could not be ruled out.
Workshop on nitrogen dioxide (NO2)
In 2011, the Department of Health commissioned a workshop on NO2, organised by Public health England (formely the Health Protection Agency) to which National and European experts, Government policymakers and advisors were invited. The aim was to develop ideas for future research that may disentangle the possible adverse health effects of NO2 from those of other air pollutants notably particles. Presentations by experts in the field and discussions by the group lead to a number of research recommendations.
Visit the Public Health England website to read the full report [external link]
US Environmental Protection Agency “Integrated Science Assessment for Oxides of Nitrogen – Health Criteria (2008)
World Health Organization (WHO) Air Quality Guidelines - Global Update 2005. Particulate Matter, Ozone, Nitrogen Dioxide and Sulphur Dioxide (2006)
Health Protection Agency (2011) HPA-CRCE-026: Report of a Workshop to Identify Needs for Research on the Health Effects of Nitrogen Dioxide - London, 2-3 March 2011