2. How does Nitrogen Dioxide (NO₂) affect human health?

2.1 Effects of long-term exposure to levels of NO₂ observed currently in Europe
2.2 Is NO₂ per se responsible for effects on health?
2.3 Are health effects of NO₂ influenced by the presence of other air pollutants?
2.4 Characteristics of individuals that may influence how Nitrogen Dioxide affects them
2.5 Is there a threshold below which nobody’s health is affected by NO₂?

2.1 Which effects can be expected of long-term exposure to levels of NO₂ observed currently in Europe?

WHO states:

2.1.1 "The epidemiological studies provide some evidence that long-term NO₂ exposure may decrease lung function and increase the risk of respiratory symptoms." More...

2.1.2 "Methodological limitations constrain identification of harvesting [(the advancement of mortality by only relatively few days)] due to NO₂ itself. The few [available] long-term studies have not shown evidence for association between NO₂ and mortality. Associations have been observed between NO₂ and mortality in daily time-series studies, but on the basis of present evidence these cannot be attributed to NO₂ itself with reasonable certainty." More...

Source & © : WHO Europe (2003)

2.2 Is NO₂ per se responsible for effects on health?

The evidence for acute effects of NO₂ comes from controlled human exposure studies to NO₂ alone.
For the effects observed in epidemiological studies, a clear answer to the question cannot be given. Effects estimated for NO₂ exposure in epidemiological studies may reflect other, often unmeasured, traffic related pollutants, for which NO₂ is an indicator. Additionally, there are complex interrelationships among the concentrations of NO₂, PM, and ozone in ambient air. More...

2.3 Are health effects of NO₂ influenced by the presence of other air pollutants?

WHO states: "There have been few controlled human exposure studies on interactions with other chemical pollutants, although several studies show that NO₂ exposure enhances [allergic] responses [of asthmatics] to inhaled pollens. Some epidemiological studies have explored statistical interactions of NO₂ with other pollutants, including particles, but the findings are not readily interpretable." More...

Source & © : WHO Europe (2003)

2.4 Which characteristics of individuals may influence how NO₂ affects their health?

Are effects of NO₂ dependent upon the subjects’ characteristics such as age, gender, underlying disease, smoking status, atopy, education etc?

What are the critical characteristics?

WHO states: "In general, individuals with asthma are expected to be more responsive to short-term exposure to inhaled agents, when compared to individuals without asthma. Controlled human exposure studies of short-term responses of persons with and without asthma to NO₂ have not been carried out. There is limited evidence from epidemiological studies that individuals with asthma show steeper concentration-response relationships. Small-scale human exposure studies have not shown consistent effects of NO₂ exposure on airways reactivity in persons with asthma, even at exposure levels higher than typical ambient concentrations. As for other pollutants, children can reasonably be considered to be at increased risk. There is limited evidence for influence of the other listed factors [such as gender, smoking status, atopy, education, etc.] on the effects of NO₂." More...

Source & © : WHO Europe (2003)

2.5 Is there a threshold below which nobody’s health is affected by NO₂?

WHO states: "The evidence is not adequate to establish a threshold for either short or long-term [NO₂] exposure. While a number of epidemiological studies have described concentration-response relationships between ambient NO₂ and a range of health outcomes, there is no evidence for a threshold for NO₂." More...

Source & © : WHO Europe (2003)

See also: General Issues and Recommendations on Air Pollutants:

• question 1.3 on uncertainties in defining thresholds
• question 3.1 recommendations regarding the concept of thresholds