Natural gas in its raw form is odourless. The smell associated with domestic gas supplies in the UK is produced by a chemical called mercaptan (ethyl mercaptan or a similar compound) that is deliberately added to the gas by the distribution company specifically so that leaks can be detected by smell. This addition is a critical safety measure; without it, gas leaks could accumulate to dangerous concentrations without any warning.

Describing the Smell

The added odorant gives natural gas a smell most commonly described as similar to rotten eggs, sulphur, or occasionally compared to a very strong, unpleasant garlic odour. The smell is distinctive and not easily confused with other common domestic smells. At low concentrations, such as a very small leak in a rarely visited area, the smell may be faint and easy to dismiss. At higher concentrations, such as a significant leak in an enclosed room, it becomes very noticeable and difficult to ignore.

The smell can also be detected from outside the property if the gas supply pipe underground is leaking; this is often detected as a smell rising through the ground in a garden or on the pavement. Dead or dying vegetation over the route of an underground gas pipe can also indicate a long-term underground leak.

LPG and Propane

LPG (liquefied petroleum gas) and propane used in gas bottles also have an odorant added to them. The smell is similar to natural gas but sometimes described as slightly more sweet or petrochemical than the natural gas odorant. The same principle applies: if you smell gas from an LPG appliance or from the area around a gas bottle connection, treat it seriously.

What to Do If You Smell Gas

Do not turn any electrical switches on or off, including light switches. Do not use a mobile phone inside the property. Do not smoke or use any naked flame. Open windows and doors to ventilate. Turn off the gas at the emergency control valve near the meter if it is safe to reach. Leave the property immediately. Call the National Gas Emergency Service on 0800 111 999 from outside the property. This number is free to call at any time. Do not re-enter until an engineer has confirmed it is safe.

Summary

Natural gas smells like rotten eggs or sulphur due to the mercaptan odorant added specifically so leaks can be detected. LPG has a similar but slightly different smell. If you smell gas, do not operate electrical switches, open windows, turn off gas at the meter, leave immediately, and call 0800 111 999 from outside. Gas detectors and carbon monoxide alarms provide additional protection.