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There have been several instances of major industrial disasters related to the use of chemicals. Although they are individual accidents, different in the way in which they happened and the chemicals that were involved, they have one common feature: they were uncontrolled, involving fires, explosions or the release of toxic substances that either resulted in the death and injury of large numbers of people inside and outside of the factory or caused extensive damage to the property and the environment.

Accidents involving major hazards could start with

  • leakage of a flammable substance, mixing of the substance with air, formation of a flammable vapour cloud and drifting of the cloud to a source of ignition leading to a fire or an explosion.
  • leakage of toxic substances, formation of a toxic vapour cloud and drifting of the cloud.

These clouds would directly affect the site as well as possibly the surrounding populated areas. In the case of flammable substances the greatest danger arises from sudden massive escape of volatile liquids or gases. If the cloud were ignited, the effects of combustion would depend on many factors, such as wind speed and the extent to which the cloud was diluted. The area affected would generally be limited to a few hundred metres from the site.

Much larger areas can be dangerously affected in a sudden release or by very large quantities of toxic materials. In favorable conditions such a cloud can still contain lethal concentrations of toxic chemicals several kilometers from the accident site. The extent of casualty depends on the number of people in the path of the cloud and on the efficiency of emergency arrangements, for example, evacuation before the cloud reaches the populated areas.

The effect can also migrate into other factories situated nearby and containing flammable, reactive or toxic chemicals, escalating the disaster. This is sometimes referred to as the `domino effect'.

Not only does the cloud itself pose a health hazard, but the fires cause depletion of oxygen and fumes generated by the fire may contain toxic gases.

Chlorine and ammonia are the toxic chemicals most commonly used in quantities large enough to pose a major hazard. Both have a history of major accidents. There are also other chemicals which, although used in smaller quantities should, be handled with particular care because of their higher toxicity.

An industrial accident classified as a `major hazard' leads to tighter control, more specific than that applied in the normal factory operations. This is in order to protect both workers and outside people, to avoid economical losses to the factory and damage to the environment.

The first step in a systematic approach is to identify the installations susceptible to a `major hazard'. For this purpose, EU in Europe has a Directive which has been in use since 1984. The Directive sets certain criteria based on the toxic, flammable and explosive properties of the chemicals. For the selection of specific industrial activities which involve a `major hazard' risk, a list of substances with limit amounts is provided. The list contains 180 toxic substances whith the limits varying from 1 kg for extremely toxic substances to 50 000 tons for highly flammable liquids. (See the list in the section: `Identification, Classification and Labelling of Chemicals', Annex 7.)

Criteria for Major Hazard Installation

1. Very Toxic (Category 1 and 2) and Toxic substances (Category 3)

Substances classified to hazard categories below according to their acute toxicity.

Classification can also be done by determining the acute toxicity in animals, expressed in LD50 or in LC50 values and using the following limits

  • Substances which correspond to the first line of the table below
  • Substances which correspond to the second and third line and which, owing to their physical and chemical properties, are potential candidates for a major hazard similar to that caused by substances filling the criteria of the first line in the table.
Category LD50 absorbed orally in rat (mg/kg bodyweight) LD50 dermal absorption in rat or rabbit (mg/kg bodyweight) LC50 absorbed by inhalation in rat (mg/litre per 4 hours)

1 <5 <10 <0.10

2 5-25 10-50 0.1 - 0.5

3 25 - 200 50 - 400 0.5 - 2

2. Flammable substances

  • Gases which form flammable mixtures with air
  • Highly or extremely flammable liquids with flash points lower than 21 C
  • Flammable liquids with flash points lower than 55 C

3. Substances which may explode when in contact with a source of ignition or which are more sensitive to shock and friction than dinitrobenzene.

The industrial activities creating the risk of a major hazard may not be restricted to defined sectors. Experience has shown that such installations are most commonly associated with the following activities:

  • petrochemical works and refineries
  • chemical works and chemical production plants
  • LPG (Liquid Petroleum Gas) storage and terminals
  • stores and distribution centres of chemicals
  • large fertilizer stores
  • factories handling explosives
  • works in which chlorine is used in bulk quantities

To set priorities a shortened list of Major Hazard Chemicals is provided to be used as a guide. Priorities can also be set within the factory to identify the most hazardous areas in the production activities.

Below is a list of priority chemicals used in identifying major hazard installations.

Name of the substance Quantity EC list number

General flammable substances
Flammable gases
Highly flammable liquids

200 t
50 000 t


Specific flammable substances
Ethylene oxide

50 t
50 t


Specific explosives
Ammonium nitrate

2500 t
10 t
50 t

146 b

Specific toxic substances
Sulphur dioxide
Hydrogen sulphide
Hydrogen cyanide
Carbon disulphide
Hydrogen fluoride
Hydrogen chloride
Sulphur trioxide

200 t
500 t
25 t
250 t
50 t
20 t
200 t
50 t
250 t
100 t


Specific very toxic substances
Methyl isocyanate

150 kg
750 kg




Chlorine poisoning in Sri Lanka
A case of chlorine poisoning in a 37-year-old mechanical supervisor at a water purification plant in Sri Lanka is described. Manipulating the main cylinder valve, he was exposed to chlorine fumes for a few seconds as he was running in and out to stop the gas flow. He started to have an intense feeling of suffocation and tightness of chest, coughing, intolerable irritation of eyes and mouth, headache and stomach problems. He still had symptoms 27 days after the incident.
Transport accident
A massive chlorine release as a result of a tank leak in a car carrying chlorine took place in Norway. A total of 85 people, from 6 months to 82 years of age were hospitalized, and out of those 3 died. approximately 7-8 tons of chlorine gas formed a 10 km long cloud which covered the valley.

Facts about chlorine

It is a greenish-yellow gas with a pungent odour. Chlorine is heavier than air and the cloud formed tends to spread along the ground. It can fill cellars or flow into subway tunnels as it did in an accident in New York leading to the hospitalization of 208 persons.

Chlorine is chemically very active. Dry chlorine at ambient temperatures reacts directly with many materials including metals. Dry chlorine does not attack steel and it is supplied commercially in steel containers in liquid form under pressure.

As liquid chlorine evaporats, at boiling point (-340 C), one volume unit of liquid forms 457 volume units of gas.

Traces of moisture in chlorine lead to rapid corrosion of steel, copper and nickel. Chlorine react vigorously with organic compounds including mineral oils and greases. Mixtures of chlorine and hydrogen gases are explosive.

Chlorine dissolves in water at a rate of 6.5 g of chlorine to one litre of water at ambient temperature. The solution is acidic and corrosive, and it has oxidizing, bleaching and germicidal properties. The water solution in a process should be kept above a temperature of 9.60 C in order to avoid blockages as a result of formation of solid chlorine hydrate.

The reactivity of chlorine strongly limits the choice of materials used in construction when planning an installation. A system constructed of steel must itself be dry before allowing chlorine to enter in it. Titanium is a satisfactory construction material at temperatures well below 1000 C provided that the moisture level is kept high. Titanium is resistant only to wet chlorine, and consideration should be given to a possible fault where dry chlorine could come into contact with the titanium. Other materials which are resistant to the attack of both wet and dry chlorine gas at ambient temperatures include glass stoneware, porcelain and some plastics.

Where chlorine is part of the product, such as chlorinated hydrocarbon solvents, it may be liberated in a fire or when in contact with incompatible chemicals giving off hazardous gases and fumes.

The recommended exposure limit, Threshold Limit Value (TLV), for chlorine is 1 part per million (ppm), a concentration which is at the limit of odour detection. The Short Term Exposure Limit (STEL) is 3 ppm.

Chlorine is a respiratory irritant. Exposure to chlorine at levels of around 15 ppm leads to irritation of the mucous membranes of the eyes and nose, and especially of the throat and lungs. Liquid chlorine causes frost burns and is corrosive to human tissue.

The gas becomes fatal at concentrations of 100-150 ppm with an exposure duration of 5-10 minutes.

The accidental instantaneous release of 10 tons of chlorine may result in a maximum concentration of 140 ppm at a distance of 2 kilometres downwind from the source and 15 ppm at a distance of 5 kilometres (under normal non-inversion weather conditions).

A chlorine vessel of 1 ton releasing liquid at full flow through an open valve will be empty in about 10 minutes, and a cylinder in far less time.

Effects of chlorine gas concentrations on people (1 ppm = 3 mg/m3)

Concentration (ppm) Time Effect

3-6 - Causes burning feeling which can be tolerated, if not other ill effects, for up to 1 hour

10 1 min Coughing

10-20 30 min Dangerous-immediate irritation of nose, throat and eyes

100-150 5-10 min More vulnerable persons might die

300-400 30 min Predicted average lethal concentration for active, healthy people

1000 A few breaths Likely to be fatal

Gas filters are effective against chlorine only at low concentrations. Filter type B can be used for concentrations below 0.1 % by volume. If the colour of chlorine gas is visible the concentration exceeds the recommended exposure limit mentioned above (TLV).

The EU classification for chlorine is toxic, T, with risk phrases:

R23 Toxic by inhalation.

R36/37/38 Irritating to eyes, respiratory system and skin.

and safety phrases:

S1/2 Keep locked up and out of reach of children.

S7/9 Keep container tightly closed and in a well-ventilated place.

S45 In case of accident or if you feel unwell, seek medical advice immediately (show the label where possible).

Where is chlorine used?

Chlorine is widely used in making many everyday products. It is used for purifying drinking-water world wide and for wastewater treatment. It is also extensively used in paper and pulp production, dyestuffs, the textile industry and in petroleum production. It is found in medicines, antiseptics, insecticides, foodstuffs, solvents, paints, plastics and many other consumer products. Most of the chlorine produced is used in the manufacture of chlorinated compounds in sanitation, pulp bleaching and textile processing, and in the pesticide industry.

Transport of chlorine

During transportation, chlorine has an UN identification number 1017. It is placed in Class 2.3 with subsidiary danger classification of Class 5.1 and 8. Containers and cylinders should have the corresponding symbols and the transporting vehicle should have visible placards.

Where chlorine is used in large quantities, it is subject to several notification requirements, standards, advice, restrictions, operational codes and maintenance procedures because of the potential risks involved in its storage and handling. The advice should used to develop emergency plans and routine maintenance checklists, taking into account the special features of the specific installation.


CLAYTON G.D. and CLAYTON F.E., ed., Patty's Industrial Hygiene and Toxicology, 3rd Revised Ed., John Wiley & Sons Inc., USA 1978

82/501/EEC Council Directive of 24 June 1982 on the major accident hazards of certain industrial activities

87/216/EEC Council Directive of 19 March 1987 amending Directive 82/501/EEC on the major-accident hazards of certain industrial activities

East African Newsletter on Occupational Health and Safety, Supplement 2/ 1989, Institute of Occupational Health, Finland 1989

ILO, International Labour Organisation, Encyclopedia of Occupational Health and Safety, Vol I -III, Geneva 1983

ILO, International Labour Organisation, Major Hazard Control, A Practical Manual, 2nd Ed., Geneva 1990

ILO, International Labour Organisation, Prevention of Major Industrial accidents, Code of Practice, Geneva 1991

IPCS, International Programme on Chemical Safety, Environmental Health Criteria 21, Chlorine and Hydrogen Chloride, WHO, Geneva 1982

IPCS, International Programme on Chemical Safety and CEC, Commission of the European Communities, International Chemical Safety Cards, Chlorine ICSC#126

UNITED NATIONS, Transport of Dangerous Goods, 8th Ed., New York 1993

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Updated by AS. Approved by EC. Last update: 30.11.2004.