Nitrogen dioxide (NO2)

  • This is a reddish-brown irritant gas with a pungent, acrid smell.
  • It is a by-product of combustion (heaters, vehicles and ships). It is estimated that as much as 60% may come from motorised vehicles.
  • It causes asthma and bronchitis, especially in children, and may reduce pulmonary functions.

Fine suspended particles (PM)

  • These are small particles that float in the air and enter our bronchia and lungs when we breathe. The finest can enter and settle in our blood vessels. We distinguish between those under 10 microns (PM10) and those under 2.5 microns (PM2,5).
  • As much as 40% of these particles are generated by engines, vehicle brakes and abrasion between tyres and road surfaces. The rest comes from construction work dust and sometimes from Saharan dust clouds. They reach high levels when temperatures are low and there is no wind blowing.
  • They cause numerous nose and eye allergies. Continued exposure is a risk factor for cardiovascular illnesses and lung cancer.

Tropospheric or environmental ozone (O3)

  • This is a colourless, oxidising gas resulting from photochemical reactions between the nitrogen oxides present in environmental pollution and certain volatile organic compounds. It should not be confused with the atmosphere's upper ozone layers which protect us from the sun's UV radiation.
  • The stronger the sunshine, the higher the concentration.
  • It causes asthma and eye allergies and can worsen respiratory and heart diseases.

Volatile organic compounds (VOCs)

  • These are very volatile carbon-based molecules, because of their readiness to turn into gases. That means they can travel long distances from their point of emission.
  • They come from vehicles, industry and the use of solvents.
  • They irritate the lungs and reduce pulmonary capacity. Some VOCs are classed as carcinogens.

Carbon monoxide (CO)

  • This is a colourless, odourless insipid gas and is therefore undetectable without measuring devices.
  • It comes from vehicle exhausts and, to a lesser extent, from heaters.
  • It can cause poisoning, headaches, nausea and vomiting in high concentrations. It is extremely dangerous indoors, given that long exposure may prove fatal.

Sulphur dioxide (SO2)

  • This is a colourless gas that smells like a burnt match. Combined with water and oxygen it causes acid rain.
  • It comes mostly from thermal power plants and other industrial activities.
  • It irritates the skin and respiratory passages and, in the long term, it can cause serious pulmonary illnesses.