The short answer. Natural gas has no smell of its own, so suppliers add an odorant that smells like rotten eggs or sulphur. If you notice that smell, do not touch switches or naked flames. Open windows, leave the property, and call the National Gas Emergency Service on 0800 111 999.
A gas leak is one of those things most of us only think about in the moment it happens. By then, you want to recognise it straight away and know exactly what to do. The good news is that the smell is deliberately distinctive, and the safety steps are simple enough to remember under pressure. This guide explains what that smell actually is, why it exists, the other signs worth knowing, and the calm sequence of actions that keeps you and your household safe.
Why gas has that rotten-egg smell
In its natural state, the gas piped into UK homes is colourless and has no odour at all. On its own, it could escape without anyone noticing until it reached a dangerous level. To prevent that, gas suppliers add a tiny amount of a harmless odorant, commonly a sulphur-based compound called mercaptan, before the gas reaches your property. That odorant is what gives mains gas its unmistakable smell.
Most people describe it as rotten eggs, bad drains, or a sulphurous, slightly sour stench. Some compare it to a struck match or a faint chemical tang. The smell is intentionally unpleasant so that even a small leak grabs your attention. Even a faint whiff is worth taking seriously, because your nose can pick up the odorant long before the gas becomes a hazard.
Other signs of a gas leak
Smell is the most reliable clue, but it is not the only one. A sense of smell can fade after prolonged exposure, and some people have a weaker sense than others. It helps to know the other signals that a leak may be present.
Signs you might notice:
- A hissing or whistling sound near a gas pipe, meter or appliance
- A pilot light that keeps blowing out, or burns orange or yellow rather than crisp blue
- Sooty or yellow-brown staining around a boiler, fire or other gas appliance
- More condensation on the windows than usual, or a stuffy, unfamiliar atmosphere
- Patches of dead or dying plants near an underground gas pipe outdoors
What to do if you smell gas
If you think there is a gas leak, the priority is safety, not investigation. A gas and air mixture can ignite from the smallest spark, so the actions below are about removing every possible source of ignition and getting fresh air into the property. Work through them calmly.
Do not turn any light or power switch on or off, do not light a match or any flame, do not smoke, and do not use anything that could create a spark, including your doorbell or a mobile phone while you are still inside. Even a switch that seems harmless can be enough to ignite gas.
Do open doors and windows to let the gas disperse. If it is safe and easy to reach, turn the gas off at the meter using the emergency control valve, which usually sits beside the meter and turns roughly a quarter turn so the handle points across the pipe. Then leave the property and move everyone, including pets, outside to fresh air. Once you are at a safe distance, call the National Gas Emergency Service on 0800 111 999. The line is free and answered at any time of day or night, and the team will tell you exactly what to do next. If the smell is coming from a cellar or basement, do not go in.
Do not go back inside until a qualified engineer has confirmed it is safe. If anyone feels unwell, with symptoms such as dizziness, nausea or a headache, mention this when you call and seek medical advice.
Gas you cannot smell: carbon monoxide
There is one important danger the rotten-egg smell will never warn you about. Carbon monoxide is a separate gas, produced when a fuel such as gas, oil, wood or coal does not burn properly. It is colourless, tasteless and has no smell at all, which is why it is often called the silent killer. It comes from faulty or poorly ventilated appliances rather than a gas pipe leak.
Because you cannot rely on your nose, the symptoms matter. Headaches, dizziness, nausea, tiredness and breathlessness that ease when you leave the house and return when you go back are warning signs. A working carbon monoxide alarm is the only reliable way to detect it. If your alarm sounds or you suspect carbon monoxide, get fresh air, leave the property, and call the same emergency line on 0800 111 999. For health concerns, contact NHS 111. Having gas appliances serviced regularly by a qualified engineer is the best way to prevent it in the first place.
How professional gas leak detection works
Once the emergency service has made the property safe, the next question is usually where the leak is and what it will take to put right. That is where specialist leak detection comes in. The aim is to find the exact source with as little disruption as possible, so there is no need to lift every floorboard or strip out a kitchen on a hunch.
A trained engineer combines several non-invasive methods. Tracer gas testing introduces a safe gas into the pipework so sensitive instruments can pinpoint where it escapes. Pressure testing confirms whether a section of pipe is losing gas and helps narrow down the area. Electronic gas detectors and acoustic equipment can then home in on the precise spot, often through a wall or under a floor, without guesswork. By isolating the location accurately, the repair can be targeted, the mess kept to a minimum, and your home put back to normal sooner.
A leak that involves gas is never a job for DIY. Working on gas pipes and appliances is restricted to suitably qualified engineers for good reason. Our role is to locate the problem precisely so that the right repair can be carried out safely and with confidence. You can read more about our approach on our gas leak detection service page, or explore the wider help in our articles. We work with homeowners and businesses right across Devon.
Frequently Asked Questions
No. Mains gas is naturally odourless. The smell comes from a harmless odorant, often a sulphur compound called mercaptan, that suppliers add on purpose. It is there so that a leak is easy to notice quickly, well before the gas can build up to a dangerous level in your home.
Call the National Gas Emergency Service on 0800 111 999. The line is free and answered at any hour. Phone from outside the property once you and everyone else are safely out in fresh air, and follow the advice the team gives you.
If the meter is easy and safe to reach, turn the gas off at the emergency control valve beside it, which typically needs about a quarter turn. If it is hard to access or you would have to enter a cellar, leave it, get out of the property, and let the emergency team advise you instead.
No. Repairs to gas pipes and appliances must be carried out by a suitably qualified engineer. A specialist can locate the exact source using non-invasive methods such as tracer gas and pressure testing, so the right repair can be made safely and with far less disruption to your home.
That can be a sign of carbon monoxide, a separate gas with no smell, produced by appliances that are not burning fuel properly. Headaches, dizziness and nausea that ease when you go outside are warning signs. Fit a carbon monoxide alarm, leave the property if it sounds, and call NHS 111 for health advice.
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