Nitrogen Oxides (NOx)
NOx is the collective term for the gases nitric oxide (NO) and nitrogen dioxide (NO2). Emissions from traffic are the main source of nitrogen oxides (NOx) in Ireland, along with electricity generating stations and industry.Short-term exposure to NO2 gas is associated with adverse respiratory effects, while NOx in general contributes to the formation of ground-level ozone and acid rain.
The NO2 concentrations in Ireland were static for the period 2008‑2015, perhaps because of a combination of the economic downturn and favourable weather conditions. However, the economic recovery has seen an increase in NO2 levels in urban areas, resulting in an exceedance of the NO2 annual limit value at one Dublin station in 2019. It will be important to remain vigilant to increasing NO2 levels, particularly from transport in urban centres.
Particulate matter (PM10 and PM2.5)
There are many sources of particulate matter (particles) including domestic solid fuel burning, diesel fuelled vehicle emissions, agriculture and even natural sources such as sea salt and wind-blown dust. These small particles can penetrate the lungs and cause health effects. There are two main types - PM10 (diameter less than 10µm) and PM2.5 (diameter less than 2.5µm).
PM2.5 tends to be a better signifier of man-made pollution, whereas PM10 can have a greater contribution from natural sources. In Ireland, levels for both PM10 and PM2.5 are above the WHO air quality guidelines values. Bringing the PM levels down below the WHO guideline values will be a challenge, requiring co-operation across a number of sectors.
Polycyclic Aromatic Hydrocarbons (PAHs)
PAHs are organic compounds predominantly originating from solid fuel burning, particularly wood burning and, to a lesser extent, vehicle emissions. A reduction in the use of solid fuel as a home-heating source across Ireland would mitigate PAH impact on air quality into the future.
Ground Level Ozone (O3)
At ground level, higher concentrations of ozone in the air have adverse implications for human health, for crops and other vegetation. With respect to human health, high concentrations of ozone affect the functioning of the respiratory system. Levels in Ireland are highly influenced by transboundary sources but are low in comparison with those in mainland Europe.
In Irish urban areas, ozone is depleted through reactions with traffic-emitted pollutants; therefore levels of ozone are higher in rural areas than in urban areas. Average concentrations in Ireland are generally below the thresholds for effects on human health and vegetation set down in the Clean Air for Europe (CAFE) Directive, but can exceed the WHO air quality guideline values for ozone.
Dioxins and PCBs
"Dioxins” is a collective term for over 200 chemical compounds, of which 17 are considered to be of toxicological significance. These compounds arise mainly as unintentional by-products of incomplete or poorly controlled combustion (e.g. backyard burning of waste) and from certain chemical processes. To maintain surveillance of dioxins, the EPA conducts surveys based on levels found in cows’ milk. All dioxin levels recorded in these surveys have been well below legislative limits and compare favourably with those from previous surveys and from other EU countries.