Chemical accident
Unlike most hazards, here the rule is not "out" but "in": go to the nearest building immediately, close windows and doors and switch off ventilation and air conditioning. The general alarm – an ascending and descending howl of 1 minute – means: turn on the radio and wait for the authorities' instructions. Do not go outside.
A chemical or industrial accident (legally a "major accident") releases hazardous substances – toxic, flammable or explosive gases and liquids. Sources are industrial plants, tank farms or the transport of dangerous goods by rail and road, for example chlorine or ammonia. In Switzerland the Ordinance on Protection against Major Accidents (in force since 27 February 1991) governs prevention for such plants and transport routes.
The main danger is a spreading cloud of pollutants. Unlike many other hazards, the protective rule here is not "get out" but "get in": taking shelter in a building, closing windows and doors and switching off ventilation gives far better protection than being outdoors. The population is warned via sirens (general alarm), radio and Alertswiss, which provide specific instructions.
The Sandoz fire near Basel in 1986 showed how severe such an accident can be: with the firefighting water, large quantities of pesticides and mercury reached the Rhine and triggered a fish kill over hundreds of kilometres. The event was one of the triggers for today's major-accident prevention and the Rhine alarm system.
Risk rating
Likelihood: Medium · Impact: Medium → Overall risk: Medium.
| 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.
In a major accident, toxic gases, fires or corrosive substances can be released from industrial plants or during transport. In a chemical alarm: go indoors, close the windows, listen to the radio.
Immediate measures
- Immediately into the nearest building, close windows and doors
- Switch off ventilation and air conditioning
- Turn on the radio (SRG), wait for instructions
Warning signs
- Siren "general alarm": a one-minute rising and falling tone, repeated after five minutes
- Pungent, unusual chemical smell in the air
- Visible gas or smoke cloud, haze over an industrial site
- Irritation of eyes, airways or skin – also in several people at the same time
- Push notifications from Alertswiss and police loudspeaker announcements
Before · During · After
Before
- Know: in a chemical alarm you go indoors – not outside
- Keep a battery radio (FM/DAB+) and the Alertswiss app to hand
- Know rooms that are easy to seal off (few windows)
During
- Stay in the building, close windows and doors, ventilation/air conditioning off
- Move to higher rooms (many gases are heavier than air)
- Listen to the radio and follow the authorities' instructions exactly
- Take neighbours and vulnerable people into the building
- Do not go outside, do not drive to the accident site, phone only for emergencies
After
- Only ventilate and go outside once the authorities give the all-clear
- Handle contaminated clothing/objects as instructed
- Seek medical help in case of irritation or symptoms
This really happened
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2023 Ammonia leak in Gossau (SG)
On 31 August 2023 ammonia leaked from a refrigeration system at a Migros food-processing site in Gossau; around 70 employees were evacuated and ten people taken to hospital for checks. There was no danger to the public.
Source -
2022 Ammonia leak in Dällikon (ZH)
On 19 November 2022 ammonia leaked from the refrigeration system of a vegetable-processing company in Dällikon; around 180 people were evacuated and several taken to hospital for checks.
Source -
2018 Chlorine gas leak in Näfels (GL)
On 13 March 2018, at the Lintharena in Näfels, a technical fault released chlorine gas into an indoor pool occupied by around 80 people — mostly schoolchildren; several complained of respiratory and eye irritation, with no serious injuries.
Source -
2015 Hazardous-goods rail accident in Daillens (VD)
On the night of 25 April 2015 a freight train derailed near Daillens; around 25 tonnes of sulphuric acid leaked from a tank wagon. Other wagons carrying caustic soda and hydrochloric acid remained intact; no lasting environmental damage was found.
Source -
1986 Sandoz fire at Schweizerhalle (BL)
On 1 November 1986 a warehouse holding around 1351 tonnes of chemicals burnt in Muttenz. With the firefighting water, around 30 tonnes of pesticides and about 134 kg of mercury reached the Rhine and caused a fish kill as far as Mannheim (about 400 km). The fire triggered the tightening of major-accident prevention.
Source
Frequently asked questions
How do I prepare for a chemical accident?
Internalise the core rule: in a chemical emergency, go into the building, not outside. Keep a battery-powered radio (FM or DAB+) and the Alertswiss app ready to receive instructions. Think in advance about which rooms seal well – ideally with few windows and on an upper floor.
Why should I stay inside during a chemical accident?
The main danger is a spreading cloud of pollutants. If you close windows and doors in a building and switch off ventilation, you are much better protected than outside. Because many gases are heavier than air, higher rooms provide additional protection. Warning and instructions come via siren, radio and Alertswiss.
What was the Sandoz fire and why is it still important today?
On 1 November 1986 a chemical storage warehouse burned in Schweizerhalle near Basel. With the extinguishing water, around 30 tonnes of pesticides and mercury entered the Rhine and caused fish kills over hundreds of kilometres. The incident was a trigger for today's accident prevention (Accident Prevention Ordinance) and the Rhine alarm system.
Official sources
- Fire brigade Emergency in case of a dangerous-goods accident
- Alertswiss Behaviour during chemical events
- FOEN Major-accident prevention (StFV)