3 Nitrogen dioxide
Nitrogen dioxide (NO2) is a red-brown pungent gas that is typically formed as a result of combustion processes. It is heavier than air with a vapour density of 1.58 compared to 1.0 for standard air. The odour threshold for NO2, between 1-6 parts per million (1,880-11,280 µg m-3), is much greater than concentrations measured in ambient air in New Zealand. Nitrogen dioxide gas is highly reactive, is corrosive to metals and is a strong oxidising agent. It combines with water to form nitric acid (HNO3) and nitric oxide (NO).
Emissions of NO2 can occur directly from combustion processes and as a result of the conversion of NO gas in the atmosphere. In New Zealand, motor vehicle emissions are the main source of NO2 in urban areas.
3.1 Guideline values
The ambient air quality guideline values (Ministry for the Environment, 2002) for NO2 are:
- 200 µgm-3 (one-hour average)
- 100 µgm-3 (24-hour average).
The one-hour average guideline value was lowered from the 1994 value of 300 µgm-3 (Ministry for the Environment, 1994) as a result of increased awareness of the impacts of this contaminant.
3.2 Concentrations
In most parts of New Zealand, concentrations of NO2 are much less than the air quality guideline values for both the one-hour and 24-hour averages. The main exception is roadside monitoring in Auckland, in particular along Khyber Pass Road. At this site, NO2 concentrations regularly exceed both ambient air guideline values with maximum one-hour concentrations per year ranging from around 240–440 µgm-3. Annual maximum 24-hour average NO2 concentrations at the site range from around 115–135 µgm-3. The one-hour average guideline value has also been exceeded at an air quality monitoring site along Dominion Road in Auckland.
3.3 Health effects of nitrogen dioxide
Nitrogen dioxide is toxic to various animals as well as to humans. Its toxicity relates to its ability to form nitric acid with water in the eye, lung, mucus membrane and skin. Studies of the health impacts of NO2 include experimental studies on animals, controlled laboratory studies on humans and observational studies.
In animals, long-term exposure to nitrogen oxides increases susceptibility to respiratory infections lowering their resistance to such diseases as pneumonia and influenza. Laboratory studies show susceptible humans, such as asthmatics, exposed to high concentrations of NO2 can suffer lung irritation and potentially, lung damage.
Epidemiological studies have also shown associations between NO2 concentrations and daily mortality from respiratory and cardiovascular causes and with hospital admissions for respiratory conditions. While results from these types of studies are not consistent, some suggest adverse effects may be associated with NO2 exposure at levels below existing guideline values. For example, Burnett et al (1998) found a 4.3% increase in all-cause mortality for an increase in 24-hour average NO2 levels of 47 µgm--3. Daily NO2 levels during the study ranged from 29 to 56 µgm-3, much lower than the existing 24-hour average NO2 guideline value for New Zealand of 100 µgm-3.
An evaluation of the health impacts of NO2 concentrations carried out by the Ministry for the Environment in support of the derivation of ambient air quality guideline values concluded that the health impacts associated with low level exposure to NO2 were equivocal and that the contribution of NO2 as one of a mixture of pollutants in the ambient environment was yet to be clearly defined (Dennison et al, 2002).
3.3.1 Implications for New Zealand
The Ministry for the Environment indicates that the guideline values for NO2 for New Zealand are based on a safety factor of 50% applied to the lowest observable adverse effect level for the protection of sensitive groups including children, asthmatics and people with chronic respiratory and cardiac disorders (Ministry for the Environment, 2002). Because the maximum one-hour average NO2 concentrations measured at Khyber Pass Road are in excess of twice the guideline value, it is possible that sensitive individuals in this area will suffer health impacts as a result of NO2 exposure. It is possible that adverse health effects might also occur as a result of NO2 exposure close to other roadsides within Auckland, e.g. Dominion Road.
In other areas of New Zealand, ambient air concentrations of NO2 do not breach the guideline values. If the existing guideline values provide adequate protection to sensitive individuals then adverse health effects associated with NO2 exposure are unlikely to occur.
