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Hazard · Technical

Gas leak

If you smell gas – a smell like rotten eggs – there is one rule: no ignition source. Do not operate any light switch, phone or doorbell, because even a single spark can ignite the gas-air mixture. Open windows and doors, leave the building and only call the fire brigade on 118 once you are outside.

Natural gas is given an odorant so that leaks are noticed — the typical smell is reminiscent of rotten eggs. Liquefied petroleum gas (propane/butane) in cylinders can also escape. If you smell gas, one rule counts: no ignition source, get out, report it. The main danger is not inhalation, but the explosion.

A single spark can ignite a gas-air mixture: a light switch, the doorbell, a ringing phone, a fridge starting up. An explosion in a residential building can destroy the whole house and endanger people in the surrounding area — this has proved fatal several times in Switzerland.

A second, quieter risk is carbon monoxide (CO) from the incomplete combustion of gas, wood or coal: a colourless, odourless poison. Where heating is done with gas or an open flame, a CO detector provides protection.

Risk rating

Risk · Moderate (level 2 of 5)

Likelihood: Medium · Impact: Low → Overall risk: Low.

Risk matrix: likelihood runs vertically (top = high), impact runs horizontally (right = high). The highlighted cell shows this hazard's rating.
Impact
Low Medium High
High
Medium This hazard's rating
Low

Legend: Low (green) · Medium (yellow) · High (red). Overall risk is likelihood multiplied by impact.

Escaping natural gas or LPG is highly explosive. A single spark – light switch, doorbell, phone – can be enough.

Immediate measures

  • Do not operate switches, devices or doorbells
  • Open windows and doors, leave the building
  • Alert the fire brigade on 118 from outside

Warning signs

  • Clear smell of rotten eggs (odorant added to natural gas) — the most important warning signal
  • A hissing or blowing noise at lines, connections or gas appliances
  • Dying plants / a brown patch in the lawn above a buried gas line (indication of a leak outdoors)
  • With CO (odourless!): sudden headache, dizziness, nausea in several people at the same time, easing off outdoors — and a CO detector that goes off
  • Soot marks or a yellow instead of blue flame on gas appliances (sign of incomplete combustion)

Before · During · After

Before

  • Know the smell of gas (rotten eggs) as a warning signal
  • Locate the main valve of the gas line
  • Use a CO detector where heating is done with gas/combustion

During

  • No light, no light switch, no phone in the danger area – avoid any spark formation
  • If reachable without danger: close the main gas valve
  • Open windows and doors, ventilate thoroughly
  • Leave the building, warn the neighbours
  • Only call 118 once outside and at a safe distance

After

  • Only re-enter the rooms once the fire brigade/supplier gives clearance
  • Have the gas installation checked before putting it back into operation
  • Have the cause fixed by a professional

This really happened

  1. 2025 House explosion in Staufen (AG)

    At the end of February 2025 a residential house exploded in Staufen (AG); two people died. The investigation established a gas leak: ground movements had damaged the poorly insulated gas line, gas seeped into the cellar and ignited.

    Source

  2. 2012 Gas explosion in Pratteln (BL)

    A gas explosion in a four-storey apartment building injured seven people; part of the building collapsed and all residents had to be evacuated.

    Source

  3. 2003 Gas explosion in Steffisburg

    On 12 July 2003 a propane gas explosion completely destroyed a six-family house; three people died and nine were injured.

    Source

  4. 1991 Gas explosion in Winterthur

    On 5 December 1991 a gas explosion in the Oberwinterthur district claimed two lives and destroyed four houses.

    Source

Frequently asked questions

How do I prepare for a gas leak?

Memorise the smell of gas like rotten eggs as a warning signal and locate the main valve of your gas line so you can close it quickly in an emergency. Where heating is done with gas or an open flame, a carbon monoxide detector (CO detector) should also be in the room – CO is colourless and odourless.

Why is a gas leak so dangerous?

The main danger is not inhalation, but explosion. A single spark – a light switch, doorbell, ringing phone or starting fridge – can ignite a gas-air mixture. An explosion can destroy an entire apartment building and endanger people in the surrounding area; in Switzerland this has been fatal several times.

What is the difference between a gas leak and carbon monoxide?

Leaking natural or liquefied gas smells (added odorant like rotten eggs) and is primarily explosive. Carbon monoxide (CO), on the other hand, arises from incomplete combustion of gas, wood or coal, is colourless and odourless and acts as a poison. Signs include headache, dizziness and nausea in several people at the same time. Only a CO detector provides protection.

Official sources