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Air Pollution Nitrogen Dioxide

3. How are we exposed to Nitrogen Dioxide (NO2)?

  • 3.1 Which are the critical sources of NO2 responsible for health effects?
  • 3.2 What is the relationship between ambient levels and personal exposure to NO2?
  • 3.3 What is the health relevance and importance of short-term exposure to high peak levels or exposure in hot spots for NO2?

3.1 Which are the critical sources of NO2 responsible for health effects?

The source document for this Digest states:


In most urban environments in Europe, the principal source of NO2 is NOx from motor vehicles of all types and energy production in some places.


Nitrogen dioxide is formed in the environment from primary emissions of oxides of nitrogen. Although there are natural sources of NOx (e.g., forest fires), the combustion of (fossil) fuels has been, and remains, the major contributor in European urban areas. Over the past 50 years vehicular traffic has largely replaced other sources (e.g., domestic heating, local industry) as the major outdoor source of NOx from fossil fuel combustion, and hence NO2: earlier in western Europe, more recently in eastern Europe. Other, stationary, sources (e.g., power plants or domestic) also contribute to NOx emissions, and, therefore to outdoor concentrations of NO2 in certain areas.

In the European Union vehicular traffic contributes more than half of the emissions of NOx (416). This is more than in the United States of America, but the contribution to total NOx emissions is even higher in some European cities, based on data from the 1990s. In London, for example, road transport contributes 75% of NOx emissions (417). Due to their characteristics (low emission heights; high emission densities in urbanized areas), traffic emissions are often the dominating source of urban outdoor NO2 exposure.

Source & ©: WHO Regional Office for Europe  "Health Aspects of Air Pollution" (2003), Chapter 7 Nitrogen dioxide, Section 7.2 Answers and rationales, Question 10

The same information on
Particulate MatterOzone

3.2 What is the relationship between ambient levels and personal exposure to NO2?

The source document for this Digest states:

Can the differences influence the results of studies?


In any particular setting the answer will depend on the relative contributions of outdoor and indoor sources and on personal activity patterns. A direct relationship between personal exposure and outdoor concentrations is found in the absence of exposure to indoor sources such as unvented cooking or heating appliances using gas, and tobacco smoking. However, since outdoor NO2 is subject to wide variations caused by differences in proximity to road traffic and local weather conditions, the relationship of personal exposure to measurements made at outdoor monitoring stations is variable. Results of epidemiological studies relying on outdoor NO2 concentrations may be difficult to interpret if account is not taken of exposure to indoor sources.


An individual’s exposure to NO2 from outdoor sources will depend largely on their proximity to vehicular traffic in space and time, given that mobile sources are the chief contributors to ambient NO2 in contemporary European cities (see Question 10). Ambient NO2 concentrations measured at fixed urban sites may not accurately reflect personal exposure to NO2 from outdoor sources, because ambient NO2 concentrations vary widely in most locales due to traffic patterns, the characteristics of the built environment, and meteorologic conditions (412). Fixed monitors are not necessarily sited with the intent of reflecting the population average exposure, and, therefore, the accuracy with which their measurements reflect population exposures may vary. This may be particularly pronounced with regard to short-term exposure in the order of days (128).

NO2 measurements from fixed-site monitors may provide better indices of exposure over longer time periods depending on where the monitors are located. For example, good relationships between personal and ambient NO2 concentrations have been observed in areas with high traffic densities (412). Such measurements, including concentrations measured at fixed residential locations (“front door” concentrations) may be particularly useful indicators of exposure to traffic-related pollution, especially when combined with data on individual time-activity patterns, traffic patterns, and other geographical information (19).

NO2 from indoor sources, e.g., gas cooking and heating, and ETS, contributes importantly to individual exposure when such sources are present, and may reduce the relationship between individual exposure and ambient concentrations measured at outdoor fixed sites (413, 414, 415). Epidemiological studies of the effects of long-term exposure to NO2 from outdoor sources need to take the possible correlation between indoor and outdoor concentrations into account in order to ensure that their effects are not confused. Fortunately, there is less need for such concern in studies of the acute effects of short-term exposure (e.g., daily time-series studies of mortality) because there is little reason to expect that concentrations of NO2 from indoor and outdoor sources would be correlated over such short-time intervals.

Source & ©: WHO Regional Office for Europe  "Health Aspects of Air Pollution" (2003), Chapter 7 Nitrogen dioxide, Section 7.2 Answers and rationales, Question 9

3.3 What is the health relevance and importance of short-term exposure to high peak levels or exposure in hot spots for NO2?

The source document for this Digest states:


Adverse health effects have been documented after short-term exposure to peaks, as well as long-term exposure to relatively low concentrations of PM, ozone and NO2. A direct comparison of the health relevance of short term and long-term exposures has been reported for PM, but not for ozone and NO2. For PM, long-term exposure has probably a larger impact on public health than short-term exposure to peak concentrations.

Some studies have documented that subjects living close to busy roads experience more short- term and long-term effects of air pollution than subjects living further away. In urban areas, up to 10% of the population may be living at such “hot spots”. The public health burden of such exposures is therefore significant. Unequal distribution of health risks over the population also raises concerns of environmental justice and equity.


Nitrogen dioxide: Short-term versus long-term
For NO2, there is experimental evidence that high concentrations increase bronchial responsiveness to inhaled allergens. A 30 minutes exposure to NO2 concentrations of 500–750 µg/m3 was shown to increase airway allergic inflammation and sensitivity to allergen exposure in subjects with mild asthma or allergic rhinitis (Tunnicliffe et al., 1994, Wang et al., 1995; Strand et al., 1997; 1998 Barck et al., 2002). A similar study conducted in the United Kingdom did not find such an effect when studying mild asthmatics at 400 µg/m3 for six hours (Jenkins et al., 1999). Svartengren et al. (2000) showed that short-term exposure to air pollution in a road tunnel enhances the asthmatic response to allergen. Allergic asthmatic subjects were exposed during rest for 30 min in a busy city road tunnel. Subjects exposed to road tunnel NO2 levels >300 µg/m3 had a significantly larger early reaction following allergen exposure, as well as lower lung function and more asthma symptoms during the late phase compared to the reference exposure. Although a sizeable proportion of the population is sensitized to common allergens (6–24% for just four major allergens in the European Respiratory Health Survey (Jean et al., 2002)), the public health significance of such increases in responsiveness is uncertain, as patients with more severe disease have not been studied. Also, the experimental studies have been conducted at concentrations that are unlikely to be reached in ambient atmospheres. As discussed in more detail in the answer to Question 4, NO2 in ambient air is part of a mixture of primary and secondary combustion products. Several studies have shown associations between NO2 and mortality or morbidity endpoints in time series studies which are independent of the associations with PM or ozone (Peters et al., 2000; Burnett et al., 1998; 1999). Associations with long-term exposure to mixtures represented by NO2 have been reported with respiratory morbidity as well as cardio-respiratory mortality endpoints (e.g. Hoek et al., 2002; Schindler et al., 1998; Brauer et al., 2002; McConnell et al., 2003). Effects of both long-term and short-term exposures to ambient mixtures of combustion products represented by NO2 are of concern. As with ozone, no analyses have been reported on the relative public health significance of short-term and long- term exposures to NO2.

Nitrogen dioxide: hot spots versus background
“Hot spots” for nitrogen dioxide will be dealt with PM in a joint paragraph (see below).

Particulate matter and nitrogen dioxide: Hot spots versus background
This question of “hot spots” relates to the relevance of spatial differences in exposures, i.e. the importance of location and proximity to emission sources. This issue is of relevance for NO2 and PM (also for other pollutants such as CO which are not being further discussed here). NO2 can be significantly elevated near sources of NOx, especially near busy roads. The same is true for PM, and then especially PM components such as elemental carbon and ultrafine particles which are considerably elevated near traffic sources. Recent evidence has shown that subjects living near busy roads (the best investigated type of hot spot) are insufficiently characterized by air pollution measurements obtained from urban background locations, and that they are also at increased risk of adverse health effects (Roemer and van Wijnen 2001; Venn et al., 2001; Hoek et al., 2002; Garshick et al., 2003; Janssen et al., 2003; Nicolai et al., 2003). It is worth noting that a significant part of the urban population may be affected. Roemer and van Wijnen (2001) estimated that 10 % of the population of Amsterdam was living along roads with more than 10 000 vehicles a day. Increased risks at hot spots raises concerns about an unequal distribution of risks connected to involuntary environmental exposures. This may affect in particular socially disadvantaged groups; a California study has shown that socially disadvantaged children have a higher chance of living close to major roads (Gunier et al., 2003).

In addition, the vast majority of epidemiological studies characterize exposure with measurements that describe urban background concentrations rather than concentrations at locations influenced by sources in the immediate vicinity. Thus, the effect estimates may not sufficiently include effects due to local hot spots. Even when measurements would be conducted near hot spots, especially busy roads, there are good indications that these hot spots are insufficiently characterized by measurement of the currently regulated PM10 metrics, not even by the contemplated PM2.5 metric. For that reason, WHO recommended already in response to the previous set of CAFE questions to give further consideration to black carbon or other measures of traffic “soot” (WHO, 2003). Also, further investigations are needed on effects of ultrafine particles (particles with a diameter smaller than 100 nm). Ultrafine particles have been shown to be greatly elevated near busy roads (e.g. Hitchins et al., 2000). Some studies have suggested adverse health effects of ultrafine particles at ambient concentrations (e.g. Peters et al., 1997); consequently, there is a need to address exposure to ultrafine particles as one of the possible PM characteristics important for the adverse effects observed at roadside “hot spots”.

Source & ©: WHO Regional Office for Europe  Health Aspects of Air Pollution - answers to follow-up questions from CAFE (2004), Question 4

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