On the 3rd March 2006, about 0.1 gallon of ethylene oxide (EO) leaked out while pumping it out of a railroad car at a plant in Hedingen in the Zurich canton, according to a police report.

A gas sensor situated at the main valve shut it off at this point automatically at 11 am. The water sprinkler installation that had been connected above the tank vehicle was also automatically activated. The corporate fire brigade doused the surroundings with water and were supported by the local fire brigade and the fire brigade from the Affoltern support post as well.

At the top right you will see the NFPA hazard diamond for ethylene oxide. The numbers from 0..4 (0= min. 4=max) demonstrate the various hazards. They are blue = health hazard, red = danger of flammability and yellow = danger of reactivity.

For our assessment we assume an amount of 0.5 litres that has leaked out and completely vaporises instantly. The following critical distances result using the current weather data from the 3rd March: 

Click to enlarge the program mask (German version)

Toxic effect:

For persons outdoors: 20 meters (= 65 ft). 

For persons indoors protected by a building: 20 meters (= 65 ft).

It is essential to point out that

	The minimal distance which the MET displays is always 20 metres (= 65 ft) even when the actual distance is specified as being smaller within the programme.
	The precipitation of ethylene oxide by water was not taken into account.
	Although ethylene oxide is undoubtedly present as a fluid at an ambient temperature of approx. 5°C (= 40°F) and a boiling point of 10°C (= 50°F), it is rapidly vaporised due to the high vapour pressure.

Interpretation

There was no hazard to the population due to this small amount of ethylene oxide that had escaped. The technical precautions including a gas sensor, automatic valve closure, and a sprinkler system have all paid off. 

Other accidents involving ethylene oxide 

	On the 24th June 1997, 1 person was killed and 59 were injured in Elkhart, Indiana, USA due to an ethylene oxide explosion. An evacuation up until a 1.6 km radius around this factory that manufactured hairsprays and deodorants was ordered.
	On the 10.06.2002, two railway vehicles derailed in Wolfratshausen, Germany with 53 tons of ethylene oxide and propylene oxide respectively. Fortunately, both tank wagons remained undamaged.
	On the 12.01.2005, a tanker with 20 tons of ethylene oxide on board overturned on the A31 nearby the location of Haren. An evacuation of an area up to 400m around the tanker involved in the accident was ordered. Fortunately, no ethylene oxide escaped.

You will find a comprehensive compilation of further accidents involving ethylene oxide here.