4. Should current NO₂ guidelines be reconsidered?

4.1 Impacts on public health of NO₂ reductions
4.2 Averaging period most relevant for NO₂ standards to protect human health
4.3 Reconsideration of the current WHO Guidelines for NO₂

4.1 Have positive impacts on public health of reductions of emissions and/or ambient concentrations of NO₂ been shown?

WHO states: "It has not been possible to study impacts of reduction in NO_x emissions or NO₂ concentrations in the ambient air because there have been no good examples of such reductions." More...

Source & © : WHO Europe (2003)

4.2 What averaging period (time pattern) is most relevant from the point of view of protecting human health?

Would additional protection be provided by setting standards for more than one averaging period for NO₂?

WHO states: "With regard to protection against acute health effects, either the peak-hour average or 24hr (daily) average NO₂ concentrations can be used as a measure of direct short-term exposure, since they are highly correlated in urban areas.

Having a longer-term guideline value is also supported by the evidence on possible direct effects of NO₂, and on its indirect consequences through the formation of secondary pollutants." More...

Source & © : WHO Europe (2003)

4.3 Is there new scientific evidence to justify reconsideration of the current WHO Guidelines for NO₂?

The current WHO guideline values for NO₂ are:

• a 1-hour level of 200 µg/m³ and
  
• an annual average of 40 µg/m³.

There is evidence from toxicological studies that long-term exposure to NO₂ at concentrations higher than current ambient concentrations has adverse effects. However, at current ambient air concentrations in Europe, uncertainty remains over the significance of NO₂ as a pollutant with a direct impact on human health. Moreover, there is still no firm basis for selecting a particular concentration as a long-term guideline for NO₂. The former group that proposed the 40 µg/m³ annual guideline value selected that value from a prior WHO review.

In recent epidemiological studies on the effects of mainly traffic generated air pollution, NO₂ has been associated with adverse health effects even when the annual average NO₂ concentration is within a range that includes 40 µg/m³. No alternative guideline could be established from these studies, therefore it was recommended that the WHO annual specific guideline value of 40 µg/m³ should be retained or lowered.

No recent human exposure study supports the need to change the 1-hour guideline value. More...