Indoor nitrogen dioxide

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Katleen De Brouwere and Rudi Torfs, VITO

Introduction and relevant exposure metrics

Indoor air quality is recognized as a priority in the European Action Plan on Environment and Health 2004-2010. The importance of indoor air quality is well understood: people spend about 90% of their time indoors, and this becomes particularly important for vulnerable groups like babies, children and aged people or people already suffering from, e.g., respiratory or allergic diseases. Indoor air contains a mix of pollutants, many of which are identified as potential health hazards. Specific studies have evaluated the effect of these pollutants on health.

More than 900 chemicals, particles and biological materials may be present in the indoor environment. The prioritization of indoor factor is essential to obtain focused and efficient research programmes and policy actions. Taking into account the conclusions of several (EU initiated) studies and actions in the domain of indoor air quality and human health (INDEX, THADE, the SCHER opinion, the EU EWGIA , WHO indoor air working group ), substances and factors which deserve high priority are environmental tobacco smoke (ETS), formaldehyde, CO, particles (PM2.5and PM10), NO2, benzene, naphthalene, moulds and mites, dampness/moisture, CO2(measure for ventilation) and radon.

the Expert Working Group on Indoor Air. Presentation on Consultative Forum on Environment & Health , 30 November 2006, Brussels

WHO working group

This scoping note will further focus on the priority pollutant NO2.

A review of strategies and protocols for indoor air monitoring of pollutants is published by Crump (2001). In general, active, pumped sampling – with normal sampling time of 30 min to 1 hour – is used for the investigation of peak or worst-case concentration, while the diffuse samplers provide a measure of the mean concentration over periods of days or weeks. The latter is passive sampling method is the most widely applied. Exposure metrics for NO2are quite straightforward: the widely applied method for indoor air NO2 sampling is a diffusion tube method, followed by colorimetrical analysis.

Indoor concentrations and personal exposures

In contrast to the assessment of ambient air concentrations, where measured or modelled concentrations at fixed-point monitors can be considered as representative for the ambient air exposure for a larger area, this is not the case for indoor air. Since indoor sources contribute significantly to the exposure, next to infiltration of outdoor or ambient NO2 with traffic as main source, individual measurements are the classical approach to assess the indoor NO2 levels in dwellings. However, recently, Baxter et al. (2007) developed a model that allows the prediction of indoor concentration of NO2 using questionnaires and knowledge of possible indoor NO2 sources and using ambient air concentrations from a centrally located ambient air monitor. This model is limited to the constraints for which the model has been developed, namely for lower socio-economic status urban households in Boston. Nevertheless, such modelling approaches should be encouraged to obtain more widely applicable models. This will be the only feasible approach if we want to setup large scale epidemiological studies (cfr. ambient air pollution studies) to investigate the impact of indoor air NO2 on human health; currently epidemiological evidence for indoor NO2 related health effects relies on smaller panels (see further).

Health effects of short-term exposures

A review of health effects of short-term exposure has recently been reported in the INDEX report (Kotzias et al., 2005). Numerous controlled clinical studies have examined lung function, airway responsiveness to pharmacological, physical (e.g. cold air) or natural (i.e. allergens) bronchoconstrictors and the defence against viral and bacterial airway infections in human subjects exposed to nitrogen dioxide. Generally, concentrations in excess of 1880 μg/m3 (1 ppm) are necessary during acute controlled exposures to induce changes in pulmonary function in healthy adults (U.S.EPA, 1993; Berglund et al., 1993; Wagner, 1985, cited in the INDEX report). Because these concentrations rarely occur in indoor air, concern about the effects of nitrogen dioxide has been focused on people with pre-existing lung disease.

The following health effects caused by NO2 exposure have been reported:

  • exposure to low levels of nitrogen dioxide can cause small decrements in forced vital capacity and forced expiratory volume in 1 second (FEV1) or increases in airway resistance in people with asthma, chronic obstructive pulmonary disease (COPD), or chronic bronchitis.
  • low-level exposures to NO2, both at rest and with exercise, enhance the response to specific allergen challenge in mild asthmatics.
  • in healthy persons, NO2 exposure has been reported to cause mild airway inflammation
  • increase in airway reactivity in sensitive humans
  • increased responsiveness to bronchoconstrictors in mild asthmatic people
  • possible detrimental respiratory effects in both normal and asthmatic subjects

Health effects of long-term exposures

To assess long-term exposures to NO2, very often proxies for exposure (NO2 indoor sources) are applied instead of long-term chemical measurements of NO2.

NO2 exposure – health relationships are one of the most widely studied among indoor pollutants. Nevertheless, evidence from three decades of epidemiological studies linking NO2 exposure to adverse health effects has been inconsistent (especially for children) (Basu et al.,1999). Indoor studies of adults have not shown a relationship between the NO2 exposure to adults and respiratory symptoms or lung functions. For children, there has been conflicting evidence between different studies. Some inconsistency may be explained by differences in methods of exposure assessment. Populations studied have also varied and include healthy children and infants, as well as children with asthma, or children/infants at risk for developing asthma (Belanger et al, 2006).

One of the studies with the most pronounced effects was the study of van Strien et al. (2004). In that cohort study, NO2 was measured in the homes of 769 infants in New England and the frequency of days with respiratory systems during their first year of life was recorded. Infants included in this study were selected based on the criteria of having a high risk of developing asthma, defined as the presence of a brother or sister in their family with asthma symptoms. Infants living in homes with homes with an NO2 concentration above the highest quartile of NO2 exposure (>17.4 ppb) had a significant higher frequency of days with wheeze, persistent cough and shortness of breath compared with homes in the lowest quartile of NO2 exposure (> 5.1 ppb). The cohort study of Neas et al. (1991) on 1159 children 7-11 years in 6 US cities between 1983 -1988 showed also a significant association of lower respiratory symptoms with indoor NO2 exposure.

Other studies showed also trends of adverse health effects of NO2 exposure in children. However, the differences in health outcomes between NO2 exposed and control groups were in many cases not statistically significant. For example, in the Danish BAMSE cohort study (> 4000 children), where children were followed during the first two years of their life, there was an increased (but statistically insignificant) risk (the odds risk > 1) for recurrent wheezing prevalence in homes above the highest quartile of indoor NO2 compared to the lowest quartile of indoor NO2 (Emenius et al., 2003).

The same trends were observed in the Australian study of Garrett et al. (1998) on 148 children 7-14 years old: the odds ratios of cough shortness of breath, waking short of breath, wheeze, asthma attacks, chest tightness, cough in the morning and chest tightness in the morning was on average above 1, however the 95 % lower confidence limit of the interval of the odds ratios was below 1 for these health endpoints.

A prospective cohort study of 1225 infants (followed from birth to 18 months) conducted in New Mexico did not show a consistent trend in incidence or duration of illness by level of NO2 exposure (Samet et al., 1993)

What health endpoints might be quantified?

A rather wide range of severe (COPD, chronic bronchitis) and milder respiratory morbidity endpoints and allergies such as cough, atopy, asthma, wheezing) (either self-reported or physician diagnosed) have been quantified.

Is there evidence of a threshold or ‘safe’ level?

Depending on the risk group. In the general population, since most known adverse health effects caused by NO2 are non-carcinogenic effects, a threshold mechanism is probably present. (needs more consideration)

The no observed adverse effect level (NOAEL) – which can be considered as a threshold value - for short term exposure are 0.47 mg NO2/m³ or increase in airway reactivity and 0.19 mg NO2/m³ for increased responsiveness to bronchoconstrictors (INDEX).

For long-term exposure, no NOAEL value could be derived, but instead the LOAEL value (lowest observed adverse effect) of 0.038 – 0.056 mg/m³ was reported for children of 5-12 years old.

Based on the review for the Environmental Health Criteria document on NO2 the WHO recommended an year average limit of 40 µg/m³ in order to protect people against long-term exposure.

The German GV II 1-week averaged guideline is 60 µg/m³.

What sub-groups of the population are most susceptible or otherwise will need special consideration in quantification?

  • For healthy adults, the risk of harmful short-term exposure is low. In contrast, effects of short-term exposure on sensitive groups such as asthmatic people and people with pre-existing lung diseases needs special consideration.
  • For long-term exposure, the group of (young) children is considered as the most susceptible group. Among them, children who have a higher risk of developing asthma (genetic predestination) deserve special attention.


  • Belanger, K., Gent, J.F., Triche, E.W., Bracken, M.B., Leaderer, B.P.2006. Association of indoor nitrogen dioxide exposure with respiratory symptoms in children with asthma. Am J Respir Crit Care Med, 173: 297–303.
  • Basu, R., Samet, J.M.1999. A review of the epidemiological evidence on health effects of nitrogen dioxide exposure from gas stoves. J. Environ. Med., 1: 173-187.
  • Baxter, L.K., Clougherty, J.E., Laden, F., Levy, J.I. 2007. Predictors of concentrations of nitrogen dioxide, particulate matter, and particle constituents inside of lower socioeconomic status urban homes. Journal of Exposure Science and environmental Epidemiology, 17(5): 433-444.
  • Crump. 2001. Review: Strategies and Protocols for Indoor Air Monitoring of Pollutants. Indoor and Built Environment, 10: 125 -131.
  • Emenius, G., Pershagen, G., Berglind, N., Kwon, H.J., Lewne, M., Nordvall, S.L., Wickman, M. 2003. NO2, as a marker of air pollution, and recurrent wheezing in children: a nested case-control study within the BAMSE birth cohort. Occupational and Environmental Medicine, 60(11): 876-881.
  • Garrett, M.H., Hooper, M.A., Hooper, B.M., Abramson, M.J. 1998. Respiratory symptoms in children and indoor exposure to nitrogen dioxide and gas stove. Am. J. Respir. Crit. Care Med., 158: 891-895
  • Kotzias et al., 2005. The INDEX project: Critical Appraisal of the Setting and Implementation of Indoor Exposure. available at INDEX%20%20EUR%2021590%20EN%20report.pdf
  • Neas, L.M., Dockery, D.W., Ware, J.H., Spengler, J.D., Speizer, F.E., Ferris, B.G. 1991. Association of indoor nitrogen-dioxide with respiratory symptoms and pulmonary function in children. American Journal of Epidemiology, 134 (2): 204-219.
  • Samet, J.M., Lambert, W.E., Skipper, B.J., Cushing, A.H., Hunt, W.C., Young, S.A., McLaren, L.C., Schwab, M., Spenger, J.D.1993. Nitrogen dioxide and respiratory illnesses in infants. American Review of respiratory disease. 148 (5): 1258-1265.
  • van Strien, R.T., Gent, J.F., Belanger, K., Triche, E., Bracken, M.B., Leaderer, B.P. 2004. Exposure to NO2 and nitrous acid and respiratory symptoms in the first year of life. Epidemiology, 15: 471-478.