Types of Hydrocarbons

There are many different types of hydrocarbons, they are a very large group of organic (carbon-based) compounds made up solely of hydrogen and carbon. They range from simple molecules like methane (one carbon attached to four hydrogen atoms) to very complex molecules made up of long chains and/or rings hydrogen and carbon atoms.

Some hydrocarbons are also classed as volatile organic compounds (VOCs) which are compounds with a boiling point of 250°C or more and they readily form a vapour in the atmosphere. Such gaseous pollutants are invisible and easily inhaled. Also, VOCs can react with nitrogen oxides in the presence of ultraviolet light (often, but not necessarily, on a sunny day) to create a form of pollution called photochemical smog - of which ozone is a key component. Ozone is a potent pollutant which can affect lung function and is especially dangerous for people with lung disease, like asthma.

The main types of hydrocarbons for concern in both indoor and outdoor air, are butadiene and benzene; which are both VOCs, and polycyclic aromatic hydrocarbons.

Butadiene (1,3-butadiene)

Butadiene is a highly flammable colourless gas, which has a petrol-like odour. It is used to make synthetic rubber for tyres and in the manufacture of plastics and various other chemicals. Leaks from industrial processes will cause butadiene to enter the environment. It is also released in vehicle exhaust fumes. Other sources include waste incineration, cigarette smoking and wood burners.

Inhalation of indoor or outdoor air containing butadiene can cause sore eyes and irritation of the nose and throat. Long-term exposures may cause damage to the heart and lungs. Exposure to higher concentrations may cause severe nervous system damage, nausea, headache, fatigue and, in extreme cases, coma. The International Agency for Research on Cancer (IARC) lists butadiene as a possible carcinogen (although exposure for low background levels is unlikely to cause serious health problems).


Another of the different types of hydrocarbons is Benzene. Benzene is a highly volatile liquid. It occurs naturally in crude petroleum at levels up to 4g/l. It is released into the atmosphere by any human activities involving petroleum and its products. Vehicle exhausts account for most of the benzene in outdoor air. Cigarette smoking (where permitted) is the main source of benzene in indoor air. Building materials like pains and adhesives may also release benzene into homes, offices and public spaces. Benzene levels are higher indoors in homes where there is a garage attached and also those close to petrol filling stations.

The AIRC classes benzene as a human carcinogen. Exposure is linked to a risk of various forms of leukaemia and aplastic anaemia.

Polycyclic aromatic hydrocarbons

The types of hydrocarbons which have molecules contain more than one ring fused together are known as polycyclic aromatic hydrocarbons (PAHs). There are several hundred different PAHs which are found as persistent environmental pollutants. They are flammable solids which are formed by incomplete combustion of organic materials, such as coal, oil, gas and wood. Other sources include vehicle exhausts, waste incineration and asphalt roads.  Polycyclic aromatic hydrocarbons are present in the atmosphere as a component of particulate pollution - which is inhaled deep into the lungs. Most environmental data refers to a PAH known as benzopyrene, which is used as an ‘indicator’ species for PAH pollution.

The main route of exposure to PAHs is from breathing indoor and outdoor air, particularly when wood smoke is around, and smoking cigarettes. The AIRC classes some PAHs, including benzopyrene, as a human carcinogen. Animal studies link PAH exposure to: cancers of the skin, lungs, bladder liver and stomach. However, the general population is unlikely to be exposed to levels high enough (on their own) to have a significant health impact.

Monitoring hydrocarbon levels

Hydrocarbons, including VOCs, are not included in the Daily Air Quality Index (DAQI) run by the Department of Environment, Food and Rural Affairs (Defra). This only covers nitrogen dioxide, sulphur dioxide, ozone and particulate matter. However, Defra does have two monitoring networks for the various different types of hydrocarbons, which generates data that is reported to the European Commission, to check compliance with environmental legislation, and is also made freely available to the public. There is an automated monitoring network that monitors butadiene and benzene (among 29 other hydrocarbons) and a non-automated monitoring network for PAHs.

Avoiding exposure to hydrocarbons

When indoors, avoid products like paint and adhesives that may contain VOCs (check the label or ask at your local DIY store). Also, don’t smoke! Make sure dwelling spaces are separate from areas where vehicles and petrol are kept, i.e. garages. Indoor use of petrol stoves and wood burning stoves may generate hydrocarbon pollution - so make sure these are properly set up and maintained to minimise the problem. Make sure your home is regularly ventilated to stop a build-up of indoor air pollution and consider using an air purifier; which can trap gaseous pollution like different types of hydrocarbons and particulate matter. Remember, outdoor air pollution can penetrate indoors and this may be an issue if you live on a busy road.

When outdoors, keep an eye on air pollution data – when ozone is high, hydrocarbons may be at high levels too – avoid cycling or walking on busy roads where pollution levels are more likely to be high. Although, there is only so much you can do on a personal level to protect against hydrocarbon air pollution.

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