Quick guide to pollutants from vehicle exhausts

Read our quick guide to pollutants from vehicle exhausts.

NOx (oxides of nitrogen)

NOx is a family of poisonous gases (called oxides of nitrogen). Exposure has been linked with increased hospital admissions due to respiratory problems, while long-term exposure may affect lung function. NOx contributes to smog, acid rain, can damage vegetation, and reacts in the atmosphere to form fine particles (see below).

Particulate matter (PM)

Visible smoke is made up of particles with a diameter of 10 microns (a millionth of a metre) or more. This is still very small: a human hair is about eight times as wide. However, particulate matter from vehicle emissions most damaging to people is smaller; often too small to see. It is between 0.1 microns and 10 microns in diameter and defined as being within a "respirable range": containing particles that can penetrate your lungs and stay there.

  • Coarse particles (between 2.5 microns and 10 microns) can lodge in your upper throat and airways to your lungs.
  • Fine particles (between 0.1 microns and 2.5 microns) are totally invisible and can reach all the way down to your lungs' alveoli (these are the tiny spaces in your lungs that allow oxygen to move into your blood).
  • Ultra-fine particles, which are even smaller, can penetrate into your blood.

Diesel engines are particularly notorious for high fine particle and ultra-fine particle emissions. Particulate matter is associated with respiratory and cardiovascular disease, including lung cancer and emphysema2.

Carbon monoxide (CO)

Carbon monoxide (CO) reduces the blood’s oxygen-carrying capacity, which can reduce the availability of oxygen to key organs. Extremely high levels of exposure are well known to be fatal, but lower concentrations of CO also pose a health risk, particularly to those suffering from heart disease.

Hydrocarbons (HC)

Hydrocarbons contribute to ground-level ozone formation leading to risk of damage to the human respiratory system. Some kinds of hydrocarbons, in addition, are both carcinogenic and indirect greenhouse gases3.

Engine noise

Vehicle engine noise is also a pollutant, the damaging effect of which is often underplayed. It causes stress to people and animals and inhibits us talking to each other as we walk and cycle and go about our daily lives.