Identifying a Gas Leak by Smell and Signs

Natural gas in its pure form is odourless and colourless. To make leaks detectable, gas suppliers add mercaptan — a chemical that produces a distinctive sulphur (rotten egg) smell at concentrations well below the dangerous range. Recognising this smell and the other signs of a gas leak, and knowing what to do immediately, can prevent an explosion or carbon monoxide incident.

The Smell of Gas

Natural gas leaks smell like:

• Rotten eggs (hydrogen sulphide)
• Sulphur
• A distinctive "gas" smell familiar to most people from their cooker pilot light

LPG (propane/butane) has a similar odorant added and smells similar — sometimes described as slightly sweet and sulphurous.

If you smell this odour:

• Take it seriously immediately — do not wait for the smell to get stronger
• Do not try to find the source by following the smell — the risk is in acting, not investigating

Other Signs of a Gas Leak

Not every gas leak is detectable by smell (low concentrations, impaired sense of smell, certain leak locations). Additional signs include:

Sign	Description
Hissing sound	Audible from a pipe or appliance; indicates pressurised gas escaping
Dead plants	Vegetation near underground pipes that dies suddenly without explanation
Dirt blowing	Soil bubbling or being disturbed near underground gas lines
White fog or mist	Not always visible, but possible near a large leak
Bubbling in water	If a garden or outdoor area has standing water near gas lines
Physical symptoms	Headache, dizziness, nausea without other explanation when in the home
Unusually high gas bill	May indicate an ongoing leak in the supply system

Physical symptoms from gas exposure arise primarily from displacement of oxygen in a poorly ventilated space, or from incomplete combustion products in adjacent appliances.

The First 60 Seconds — What to Do

If you smell gas or detect other signs of a leak:

1. Do not turn any electrical switches on or off — a spark from a light switch can ignite accumulated gas.
2. Do not use your mobile phone inside the building — call from outside.
3. Do not use any ignition source — no cigarettes, matches, lighters, or electrical appliances.
4. Open doors and windows as you move toward the exit — this ventilates the space while you leave.
5. Leave the building — immediately.
6. Shut off the gas at the meter if it is accessible on your way out and you can do so in under 10 seconds. If not, leave first.
7. Once outside, call the gas emergency number from your mobile — UK: 0800 111 999; US: your local utility emergency number.
8. Do not re-enter until the gas company or emergency services give the all-clear.

⚠️ The smell of gas, however faint, requires this response. Do not wait to see if it gets stronger. Do not try to find the source. Do not open appliances to investigate. The correct response to any gas smell is the same: get out without creating sparks, call the emergency number from outside.

Common Confusion: What Is Not a Gas Leak

Some smells are confused with gas leaks:

• Sewage / drain smell — similar sulphurous quality; if the smell is coming from drains, check p-traps (dried-out drains can allow sewer gas into the building)
• Skunk odour — mercaptan is also used in skunk spray; very similar smell
• Propane grill smell — cooking on a propane grill outdoors is normal; if you smell propane inside, it is not

If uncertain: treat it as a gas leak. The cost of evacuating unnecessarily is zero compared to the cost of staying in a building with a real leak.

Impaired Sense of Smell

Not everyone can reliably smell gas odorant:

• Elderly individuals may have reduced olfactory sensitivity
• Certain medications impair sense of smell
• Head colds and nasal congestion reduce detection ability
• Olfactory fatigue — after prolonged exposure, you may stop noticing the smell

If anyone in your household has reduced smell sensitivity, ensure they know the other indicators and that the household has functional CO and combustible gas detectors.

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Quick Reference

Sign	Action
Smell of rotten eggs / sulphur	Treat as gas leak — leave immediately
Hissing from pipe or appliance	Leave; do not operate any switches
Physical symptoms (headache, nausea) in home	Open windows while leaving; call gas emergency from outside
Once outside	Call gas emergency number; stay outside until cleared
Do not	Turn switches; use phone inside; re-enter
Uncertain	Treat as real — evacuate and call