Residential Indoor Air Quality Guideline: Nitrogen Dioxide
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Nitrogen dioxide (NO2) belongs to the nitrogen oxides (NOx) family of compounds. It is a reddish-brown gas with low water solubility, found in both indoor and outdoor air. This contaminant was identified in the Health Canada 1987 Exposure Guidelines for Residential Indoor Air Quality as the NOx species that could have adverse health effects at concentrations potentially encountered in indoor air (Health Canada 1987). The present document reviews the epidemiological, toxicological, and exposure research on NO2 that has been published since 1987, and proposes new short- and long-term indoor air exposure limits.
Sources and Exposure
Nitrogen dioxide in the indoor environment is the result of both infiltration of ambient NO2 and NO2 produced by combustion sources within the home. Major anthropogenic sources of ambient NO2 include emissions from vehicles, aircraft, locomotives, fossil fuel power stations, industrial processes, and building heating systems. Potential indoor sources of NO2 include gas, wood or kerosene appliances such as furnaces, space heaters, stoves, and water heaters. Emissions from these appliances are minimal when the appliance is well vented (i.e. exhaust gases are effectively evacuated outdoors). However, these emissions may become significant if the appliance is unvented or poorly vented. In the case of gas stoves, the degree of venting is variable as this depends on the presence and efficacy of the range hood exhaust fan as well as the extent to which residents use the fan while cooking.
Levels of indoor NO2 vary considerably between homes, due to differences in exterior and interior sources. In studies of homes in Canadian cities (Halifax, Hamilton, Regina, Windsor, Edmonton, and Toronto), median indoor levels of NO2, measured in either summer or winter, generally varied between 4 and 10 µg/m3. When only homes with gas stoves were considered, median values ranged from 9 to 22 µg/m3, with the highest levels measured in winter. The full range of concentrations measured in these studies varied from less than 1 to approximately 90 µg/m3.
Health effects of exposure to NO2 have been examined in toxicological, epidemiological, and controlled human exposure studies. In this assessment, the short-term exposure limit is derived from the results of controlled human exposure studies, whereas the long-term exposure limit is based on epidemiological data from studies conducted in homes or schools. Supporting evidence is provided by the results of epidemiological studies of the health effects of ambient NO2, and toxicological data obtained from studies conducted in experimental animals.
Controlled human exposure studies
In general, controlled human exposure studies in healthy adults suggested that respiratory and cardiovascular systems were not adversely affected by inhalation of up to 1880 µg/m3 NO2 for one to six hours, with or without exercise (Gong et al., 2005; Frampton et al., 2002; Vagaggini et al., 1996; Jorres et al., 1995; Kim et al., 1991; Rubinstein et al., 1991; Frampton et al., 1989a, Frampton et al., 1989b; Adams, Brookes and Schelegle, 1987; Folinsbee et al., 1978; Morrow et al., 1992; Frampton et al., 1991; Hazucha, Ginsberg and McDonnell, 1983; Bylin et al., 1985). However, evidence of slight hematological, inflammatory, and immunological effects was observed in some healthy adults with exposure to 1100 µg/m3 NO2 (Frampton et al., 1989a; Frampton et al., 1989b; Frampton et al., 2002).
Multiple studies among asthmatics and adults with chronic obstructive pulmonary disease (COPD) reported adverse respiratory effects of NO2 at concentrations as low as 500 µg/m3 (Vagaggini et al., 1996; Morrow et al., 1992, Roger et al., 1990; Bauer et al., 1986; Avol et al., 1989; Strand et al., 1996; Bylin et al., 1988). Asthmatic children and adults exhibited decreased lung function and/or airway hyperresponsiveness (AHR) following bronchial challenge (i.e., administration of bronchoconstricting agents) (Bauer et al., 1986; Avol et al., 1989). Furthermore, asthmatics with allergies displayed decreased lung function (Strand et al. 1997, Jenkins et al. 1999, Tunnicliffe, Burge and Ayres 1994) or increased pulmonary inflammation when NO2 exposure was followed by exposure to an allergen (Barck et al. 2002; Barck et al. 2005, Wang et al. 1995a. Wang et al. 1995b). Adults with COPD displayed decreased lung function in response to NO2, but other health effects were not observed (Vagaggini et al. 1996; Gong et al. 2005).
Evidence for the respiratory health effects of NO2 below concentrations of 500 ug/m3 is inconsistent. A small number of studies demonstrated that exposures below 500 µg/m3 could result in decreased lung function in asthmatic adults following bronchial challenge (Orehek, Massari and Gaynard 1976; Kleinman et al. 1983; Hazucha, Ginsberg and McDonnell, 1983), while other studies failed to demonstrate this effect (Bylin et al., 1985; Bylin et al., 1988; Roger et al., 1990; Jorres and Magnussen, 1991). The marked response of some individuals to NO2 with bronchial challenge suggests a large variability in the population, even among asthmatics. However, this responsiveness was not correlated with asthma severity or sensitivity to a given bronchoconstricting agent. Overall, the current evidence does not suggest age- or gender-sensitivity to NO2, although few studies have specifically evaluated this question in older adults or asthmatic children.
Indoor epidemiological studies
In terms of chronic exposure, numerous epidemiological studies have found positive associations between frequency of respiratory symptoms (e.g., wheezing, chest tightness) and long-term exposure to NO2 in the home. However, these same studies generally report little or no effect of NO2 on lung function parameters. Positive associations between indoor NO2 and respiratory symptoms were most consistently observed in studies of asthmatic children exposed to indoor NO2 concentrations that were approximately two to three times higher than those typically measured in Canadian homes.
Two randomized intervention studies support a relationship between decreased exposure to NO2 and its co-pollutants and improvement in respiratory symptoms, particularly in asthmatic children (Pilotto et al. 2004, Marks et al. 2010). In these studies, the intervention involved replacement of an unvented gas heater with a vented gas or electric heater. Studies investigating the relationship between personal NO2 exposures and respiratory health outcomes also support an association between chronic NO2 exposure and adverse effects.
A large number of studies have investigated respiratory health in relation to the presence or use of a gas stove, without direct measurement of NO2 levels. Cross-sectional and longitudinal studies have produced mixed results, with some indication of a relationship between increased respiratory symptoms and slight decreases in lung function in children when gas stoves are present in the home (reviewed in WHO 2010). The potential for exposure misclassification is greater in these types of studies as NO2 is not directly measured; this may explain in part the inconsistencies in the database.
Residential Indoor Air Quality Guideline for Nitrogen Dioxide
The determination of the Residential Indoor Air Quality Guideline (RIAQG) is carried out in two stages. First, a reference concentration (RfC) is derived by applying uncertainty factors to the concentrations at which the most sensitive adverse health endpoint was observed. The RfC represents the indoor air concentration below which individuals (including sensitive subgroups) may be exposed and not experience adverse health effects. For the short-term RfC, the exposure period is specified--in the present case, one hour. For the long-term RfC, the exposure is considered to occur over months or years, up to a lifetime.
In the second stage, the short- and long-term RfCs are compared with measured exposures in residential indoor air, and evaluated with respect to their technical feasibility. If the RfC is considered attainable where reasonable control measures are followed, the RIAQG is set equal to the RfC. If the RfC is considered unattainable with currently available risk management technology and practices, the RIAQG may be set at a higher concentration. This allows a RIAQG to be used as an achievable target for improving indoor air quality when evaluating risk management measures.
Setting the RIAQG at a higher concentration than the RfC results in a smaller margin of exposure between the RIAQG and the concentration at which effects have been observed in health studies. Nonetheless, the RIAQG is still considered protective of health, given the precautionary nature of the risk assessment process in which uncertainty factors are applied to the most sensitive adverse health endpoint observed in a sensitive subpopulation.
Short-term Residential Indoor Air Quality Guideline
For the derivation of the short-term (one hour) RfC, a point of departure of 500 µg/m3 NO2 was selected, based on effects in asthmatics in most short-term controlled exposure studies. The health effects observed at this level of exposure were decreased lung function and increased inflammation. This point of departure is also consistent with decreased lung function observed in subjects with COPD exposed to 560 µg/m3 NO2. However, it should be emphasized that there were individuals who were more responsive in some studies, suggestive of a large variability in the population.
In determining the need for an uncertainty factor (UF) for intraspecies variability, consideration was given to the increased AHR of a few sensitized asthmatics at doses as low as 190 µg/m3 NO2. Similarly, consideration was given to the uncertainty in the effects that might be observed in adults with COPD and in asthmatic children if they had been tested at concentrations of less than 500 µg/m3. An intraspecies UF of 3 is considered protective of potentially sensitive individuals (i.e., responders, adults with COPD, asthmatic children). A composite UF of 10 (3 for use of an adverse effects level as the point of departure, and 3 for intraspecies variability) was therefore applied to the short-term lowest observed adverse effect level (LOAEL) of 500 µg/m3 to obtain an RfC of 50 µg/m3.
Evaluating the feasibility of the short-term RfC for the Canadian population is limited by the lack of data on short-term peak concentrations. However, a California study of modelled indoor NO2 concentrations indicates that less than 25% of homes with gas stoves and moderately efficient hood ventilation would meet a limit of 50 µg/m3. By comparison, 75% of homes with gas stoves and moderately effective stovetop ventilation would be able to meet a limit of 170 µg/m3. For risk management purposes, a short-term RIAQG of 170 µg/m3 is therefore recommended. This proposed value would supersede the previous 1987 Health Canada short-term indoor air exposure limit of 480 µg/m3. Nearly all homes will be able to meet the proposed guidelines, although some with a gas stove may exceed the short term guideline for brief periods of time after cooking.
Long-term Residential Indoor Air Quality Guideline
For the derivation of the long-term RfC, consideration was given to the strength of the epidemiological evidence for an association between chronic indoor NO2 exposure and adverse respiratory effects, the level of exposure at which studies begin to show significant increases in effects (i.e., point of departure), and the UFs that should be applied to the point of departure. A point of departure of 30 µg/m3 was selected, based on respiratory symptoms observed in epidemiological studies of asthmatic children and supporting evidence from intervention studies. A default UF of 3 was retained to account for the fact that the point of departure is based on observed adverse effects. As the studies on which the point of departure is based are conducted in the sensitive subpopulation of asthmatic children, no further UF for intraspecies variability was employed. A UF of 3 was therefore applied to the long-term point of departure of 30 µg/m3 to obtain a long-term RfC of 10 µg/m3.
The available evidence indicates that an exposure level at the long-term RfC of 10 µg/m3 does not result in adverse health effects in the general population, including the more vulnerable subgroup of asthmatic children. However, data on gas stove homes in Canada suggest that approximately 90% of these homes would exceed an average concentration of 10 µg/m3 NO2. Moreover, approximately 10% of electric stove homes would exceed this concentration, even in the absence of a significant indoor source of NO2. For this reason, the long-term RfC of 10 µg/m3 was not retained as the long-term RIAQG.
For risk management purposes, a value of 20 µg/m3 is proposed as the long-term RIAQG. This proposed value would supersede the previous 1987 Health Canada long-term indoor air exposure limit of 100 µg/m3. Data from Canadian indoor air studies indicate that the concentration of NO2 in most electric stove homes will rarely exceed this level and that this concentration is also attainable in gas stove homes when adequate stovetop ventilation is used. Moreover, the epidemiological evidence is not suggestive of appreciable health effects associated with long-term exposure at this concentration. The recommended long-term RIAQG of 20 µg/m3 is thus considered protective of health.
When comparing a measured NO2 concentration with the long-term exposure limit, the sampling time should be at least 24 hours. However, given the fluctuation in NO2 levels throughout the day, month, or season, longer sampling periods will provide a more representative estimate for evaluating NO2 exposure occurring over months or years.
|Exposure period||Concentration||Critical Effects|
|Short-term||170||90||Decreased lung function and increased airway responsiveness in asthmatics|
|Long-term||20||11||Higher frequency of days with respiratory symptoms and/or medication use in asthmatic children|
Strategies for reducing exposure to NO2 indoors include controlling indoor emissions from combustion appliances and reducing infiltration of NO2 from adjacent sources. Control measures include the following:
- Properly install and maintain combustion appliances used for heating (e.g. gas and oil furnaces, wood stoves, gas water heaters), with venting outside.
- Use a higher fan setting when cooking on a gas stove, ensure that it vents outside, and preferentially use the back burners.
- Do not use gas-, propane-, or kerosene- based equipment in poorly-ventilated enclosed spaces.
- Do not idle cars or use combustion-powered equipment in attached garages.
- Barbeque outdoors and away from open doors and windows.
Use of these strategies will reduce exposure to NO2 and other contaminants in combustion gases, including carbon monoxide, fine and ultrafine particulate matter, and volatile organic compounds.
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