CHEMICALS

Risk Assessment for Physico-chemical Hazards in the Workplace

1. Introduction and conclusions

One has only to ponder the workings of the internal combustion engine to realise that hazardous chemical properties, such as flammability and explosivity, are highly beneficial for society.

However, in the wrong place, and in uncontrolled conditions, chemical fire and explosion can bring misery to communities and very public shame to the chemical industry. Major chemical incidents of fire and explosion are rare. However, as more information becomes available on the causes, employers have moral, financial, and increasingly onerous legal obligations to prevent them. The UK government reports the number of fires in non-domestic buildings to be over 33 000, resulting in 38 deaths, over 1300 injuries, and at a cost of £2.5 billion (2004; England and Wales). (Ref 1)

Employers using and storing hazardous chemicals are asked to assess both the likelihood and the consequences, from the benign to the disastrous, of things literally going up in smoke. They have a legal obligation to protect their workforce, and the wider public, from the potential effects of the chemicals they handle.

Prevention is the keyword for risks arising from physico-chemical properties, but the employer is also obligated to make plans should an accident occur. The employer is obliged to take a close look at the hazardous properties of the chemicals he or she is handling, and the operations that take place on the premises, and put in place effective measures for the prevention of fire and explosion.

Outlined below are some considerations for the risk assessment for physico-chemical hazards, and the legal instruments in place to ensure the safe use of chemical products in the workplace.

2. Risk assessment or risk management?

Assessing the hazards from physico-chemical properties of chemicals is fundamentally different from health and environmental risk assessment. Firstly, because many physico-chemical properties are descriptive of the chemical, rather being an indication of hazard. The ‘base-set’ of tests, required for the marketing of a new chemical in the EU, comprises the physico-chemical tests given in the following table:

Table 1 Physico-chemical information required for notification of a new chemical substance

Hazard related	Non-hazard related
Flash point (liquids)	Melting point/freezing temperature
Flammability (solids)	Boiling temperature
Flammability (on contact with water)	Relative density
Pyrophoric properties (on contact with air)	Vapour pressure
Explosive properties	Surface tension
Auto-ignition temperature (for liquids)	Water solubility
Relative self-ignition temperature (for solids)	Partition coefficient (octanol/water)
Oxidising properties	Granulometry (for solids)
	Hydrolysis as a function of pH
	Soil adsorption coefficient

If risk assessment is the ultimate purpose of chemical testing, why do we test for non-hazard-related properties at all? The answer is that the non-hazardous properties have a vital role in modelling the exposure to chemicals. For example, a high vapour pressure may indicate a higher likelihood of inhalation exposure in the workplace, and also affects the distribution of the chemical to the atmosphere in environmental modelling, for example following an accidental release.

Furthermore, for hazard-related physico-chemical properties, the classic risk assessment model (risk = hazard × exposure) does not apply, as the hazard (fire, explosion) occurs only under specific circumstances (eg concentration of vapour), there is usually no acceptable level of risk (ie any fire or explosion is unacceptable), and the effect (eg burns) are not specific to the chemical substance. We therefore use the terms risk management or risk reduction to describe the procedures for minimising the likelihood of adverse effect resulting from hazard-related physico-chemical properties.

3. Physico-chemical hazards explained

For the sake of brevity, only flammability, explosivity, and oxidising properties will be considered further, as these are the most common physico-chemical hazards likely to be encountered.

Suppliers are obligated to inform their customers on the hazardous properties of their chemical products. The label on the container gives an indication of the hazards as a symbol, accompanied by risk phrases (R-phrases), and safety phrases (S-phrases), while more detailed information on the hazards, and precautions for safe handling, are found in the product safety data sheet.

In order to assess the workplace for physico-chemical hazards, it is necessary to understand some of these properties in more detail, and how the classification of chemicals (eg explosive, flammable) is made.

3.1 Flammability

In order to create a fire there has to be fuel (eg petrol), an oxidizer (eg oxygen in air) and a source of ignition energy (eg, heat or an electrical spark). These three items are commonly referred to as ‘the fire triangle’ and represented as below.

In European legislation, flammability is divided into a number of sub-categories. The two most influential chemical classification systems in Europe are those for transport and supply. The systems use completely different symbols and codes, but the classification criteria are similar (see Table 2), with minor variations, which can lead to confusion over whether a particular substance is flammable or not. Ref 2.

Table 2 Flammable substances according to transport and supply legislation

Transport classification (see a below)		Supply classification (see b below)
Designation	Definition	Designation	Definition
Class 3 flammable liquids: Packing group III	23 ≤ fp ≤ 61 °C, bp > 35 °C	Flammable (R10)	Liquid substances and preparations having a low flash point (21 ≤ fp ≤ 55 °C) and sustained combustion
Class 3 flammable liquids: packing group II Class 4.1 flammable solids – packing group determined by ‘burn trough test’ Class 4.2 Class 4.3	fp < 23 °C, bp > 35 °C Solids that are readily combustible or ignitable through friction Substances liable to spontaneous combustion Substances which in contact with water emit flammable gases	Highly flammable (R11) Highly flammable (R11) R17 Spontaneously flammable in air R15 (contact with water liberates extremely flammable gas)	Liquid substances and preparations having a very low flash point (fp < 21 °C, but not extremely flammable) Solid substances and preparations which may readily catch fire after brief contact with a source of ignition and which continue to burn or to be consumed after removal of the source of ignition Substances and preparations which may become hot and finally catch fire in contact with air at ambient temperature without any application of energy Substances and preparations which, in contact with water or damp air, evolve highly flammable gases in dangerous quantities, at a minimum rate of 1 litre per kilogram per hour
Class 3 flammable liquids: packing group I Class 2.1 flammable gases	fp ≤ 61 °C, bp ≤ 35 °C Any gas which is ignitable when in a mixture of 13% or less by volume with air; or any gas with a flammable range with air of at least 12% regardless of the lower flammable limit	Extremely flammable (R12)	Liquid substances and preparations having an extremely low flash-point and low boiling point (fp < 0 °C; bp ≤ 35 °C) and gaseous substances and preparations which are flammable in contact with air at ambient temperature and pressure

a See the UN ADR model regulation. (Ref 3)
b See the Dangerous Substances Directive (67/548/EEC); fp = flash point; bp = boiling point.

The testing methodology for determining hazardous properties is given in separate guidance documents for transport of dangerous goods (Ref 4) and supply of dangerous substances and preparations (Ref 5).

3.2 Explosivity

The definitions for explosivity are given in the following table.

Table 3 Definitions of explosive substances

Transport classification		Supply classification (see a below)
Designation	Definition	Designation	Definition
Explosive substances Substances are divided into six Divisions (1.1 to 1.6) in approximate decreasing order of hazard	Solid or liquid substances (or mixtures of substances) capable by chemical reaction of producing gases at such a temperature and pressure and at such a speed as to cause damage to the surroundings. Pyrotechnic substances are included.	Explosive: R2 Risk of explosion by shock, friction, fire or other sources of ignition; R3 extreme risk of explosion by shock, friction, fire or other sources of ignition	Solid, liquid, pasty or gelatinous substances and preparations which may also react exothermically without atmospheric oxygen thereby quickly evolving gases, and which under defined test conditions, detonate, quickly deflagrate or upon heating explode when partially confined.

a See the Dangerous Substances Directive (67/548/EEC)

Detonation of explosive materials can occur through heating them in a confined space (eg from a warehouse fire), friction (eg grinding processes), impact, or electrical discharge. Because it is less common for the chemical industry to deal with explosive substances, I do not include a detailed treatment of risk reduction measures here. The safety assessor should seek specialist advice, perhaps from the supplier, if he or she suspects a chemical used in the workplace might have explosive properties (eg from the label). Some chemical functional groups that raise concern for explosivity are given in Table 4. The oxygen balance calculation, and thermodynamic information on decomposition of the substance can also be used as alerts for explosivity.

Table 4 Some chemical groups indicative of explosive properties

Structural feature	Chemical classes	Examples
C–C unsaturation	Acetylenes, acetylides, 1,2-dienes	Dichloracetylene
C–metal, N–metal	Grignard reagents, organolithium compounds	tert-Butyllithium
Contiguous oxygen atoms	Peroxides, ozonides	Dibenzoyl peroxide
N–O bonds	Hydroxylamines, nitrates, nitro compounds, nitroso compounds, N-oxides, 1,2-oxazoles	1,3,5-Trinitrobenzene; 2,4,6-trinitrotoluene; 2,4,6-trinitrophenol; ethyl nitrite; ethyl nitrate; cellulose nitrate
N–halogen	Chloramines, fluoramines	Nitrogen triiodide
O–halogen	Chlorates, perchlorates, iodosyl compounds	Ammonium perchlorate
Contiguous nitrogen atoms	Azides, azo compounds, diazo compounds, hydrazines	3-Azidosulfonylbenzoic acid; lead azide
Strained ring structure	Cyclopropanes, aziridines, oxiranes, cubanes	R-2,3-Epoxy-1-propanol

3.3 Explosive Atmospheres

A safety assessor will assume that flammable substances will, when dispersed in air, form an explosive atmosphere, unless there is evidence to the contrary. Where a potentially explosive atmosphere has formed, an ignition will often cause a fire rather than an explosion, depending on the conditions. The precautions for preventing an ignition of fire and explosion are often the same. Sometimes, a fire may lead to subsequent explosion, and vice versa.

For any mixture of a combustible gas or vapor with an oxidizer there is a critical ignition energy above which there will be a self-propagating explosion. Also, the air–fuel mixture will only be explosive over a range of fuel concentration. These limits, derived by experiment, are referred to as the lower explosive limit (or LEL) at the lower concentration limit, and the upper explosive limit (or UEL) at the higher concentration limit. Because the LEL depends on the conditions of the experiment, especially the size of the vessel and the energy available from the ignition source, a safety assessor would usually control the explosive atmosphere at 25 to 50% of the LEL.

Explosions are particularly nasty because, in addition to direct explosion effects, they create noxious reaction products, and cause life-threatening depletion of oxygen.

3.4 Oxidising properties

Transport classification		Supply classificationa
Designation	Definition	Designation	Definition
Oxidising substances (Class 5.1)	Substances which, while in themselves not necessarily combustible, may, generally by yielding oxygen, cause or contribute to the combustion of other material, and articles containing such substances	Oxidising R8, contact with combustible material may cause fire; R9 explosive when mixed with combustible material	Substances and preparations which give rise to a highly exothermic reaction in contact with other substances, particularly flammable substances

Oxidising substances allow materials to burn that would otherwise not be capable of sustained combustion, and increase the burning rates of combustible materials. They add to the severity of a fire by increasing the supply of oxygen (see fire triangle above). If test data is not available, it is possible to assess a chemical’s potential for having oxidising properties from its structure and oxygen balance.

4. Legal Considerations

4.1 General Provisions

There are a number of legislative measures which obligate the employer to reduce the risk of fire and explosion. In the UK, new legislation (Ref 6) and guidance1 outlines the fire risk assessment strategy comprising five stages: (1) identify fire hazards; (2) identify people at risk; (3) evaluate, remove, reduce and protect from risk; (4) record, plan, inform, instruct and train; (5) review. If an organisation employs five or more people, the employer must assign a ‘responsible person’ to carry out the risk assessment. The guidance provides good, up-to-date advice on fire safety, including storage of flammable materials, firefighting equipment and facilities, escape routes, and emergency planning.

More specifically for users of chemicals, The Chemical Agents Directive (98/24/EC) (Ref 7) obligates EU employers using dangerous chemicals to a sequence of priorities (most important first):

To prevent the presence at the workplace of hazardous concentrations of flammable substances or hazardous quantities of chemically unstable substances
Where this is not possible due to the nature of the work, to avoid the presence of ignition sources, which could give rise to fires and explosions, or adverse conditions which could cause chemically unstable substances or mixtures of substances to give rise to harmful physical effects; and
To mitigate the detrimental effects to workers in the event of fire or explosion

4.2 Explosive atmospheres and ATEX

There are two main legislative instruments for controlling explosive atmospheres, called the ATEX (ATmosphère EXplosibles) Directives. The first, the ATEX Workplace Directive, (Ref 8) obligates the EU employer to identify areas of the workplace where an explosive atmosphere might form, requiring special precautions over sources of ignition.

There are many variables in deciding whether a particular work area should be assigned as non-hazardous or hazardous, with regards to explosive atmospheres, but the following parameters need to be taken into consideration:

The hazardous properties of the substances, eg flash points and vapour density for liquids; particle size and density for solids.
The size of potential releases
Temperature and pressure of processes involving the dangerous substances
Ventilation types (natural, or local exhaust)
Travel of potential leaks (eg through ducting)

Hazardous areas are further classified into Zones, depending on the explosion potential (Table 5). The coding of the Zone determines the type of equipment that may be used in that area. The specifications of the equipment are given in the second of the ATEX Directives, the ATEX Equipment Directive (Ref 9). Equipment suitable for use in explosive atmospheres is given a distinctive ‘Ex’ symbol, and a coding to distinguish the Zone for which it is suitable.

Table 5 Zone description given in the ATEX Directive

Zone	Description A place in which an explosive atmosphere consisting of a mixture with air of flammable substances in the form of gas, vapour or mist is…	Category of equipmenta
0	…present continuously or for long periods or frequently	1
1	…likely to occur in normal operation occasionally	1 or 2
2	…not likely to occur in normal operation but, if it does occur, will persist for a short period only	1,2 or 3
Zone	Description A place in which an explosive atmosphere in the form of a cloud of combustible dust in air is…	Category of equipmenta
20	…present continuously or for long periods or frequently	1
21	…likely to occur in normal operation occasionally	1 or 2
22	…not likely to occur in normal operation but, if it does occur, will persist for a short period only	1,2 or 3

The employer should assess the explosion risk and draw up an explosion protection document, which includes the identification of the hazards, the evaluations of risks and the specific measures to be taken to safeguard the health and safety of workers. The employer is also required to have an action plan in case an explosion occurs. The plan should include provision of alarms, specialised equipment, escape and rescue plans, first aid facilities, and chemical information (eg safety data sheets) for employees and emergency services. Drills and training for the action plan should take place at regular intervals.

4.3 Seveso Directive

The Seveso Directive (Ref 10) was drafted in response to a major incident occurring near Milan, Italy. An explosion at the ICMESA chemical plant resulted in the release of the highly toxic 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) over a highly populated suburb. The legislation is aimed at preventing major incidents at chemical factories, and reducing the effects should such an incident occur.

For establishments storing large quantities (see Table 6) of reactive chemicals, EU operators are obliged to maintain a major-accident prevention policy. This document includes the overall aims and principles of action with respect to the control of major-accidents.

Upper tier establishments (see Table 6) require, in addition, a Safety Report, a Safety Management System and an Emergency Plan. The information requirement for each of these documents in given in the Annexes to the Directive. The key document for upper tier establishments is the Safety Report, which must be submitted to an Authority for examination. The Authority is obliged not only to ensure that the risk assessment and protection measures in the Safety Report are adequate for the safety of the chemical facility, but also to ensure that local residents are informed of the potential danger, and what to do in the event of a major incident. The European Commission receives a report of all major incidents, and therefore has an overview of the causes, lessons learnt, and effective measures in the prevention of such accidents.

A description of the risk assessment for major establishments is beyond the scope of this short article, but guidance on the compilation of safety reports is available (Ref 11).

Table 6 General threshold limits for the Seveso II Directive

Category of dangerous substance	Lower Tier threshold (t)	Upper Tier threshold (t)
Oxidising	50	200
Explosive (transport division 1.4; slight risk of explosion)	50	200
Explosive (other transport divisions; supply classification R2 or R3)	10	50
Flammable (supply classification R10)	5000	50 000
Highly flammable (supply classification R17; flammable substances and preparations processed under high pressure or temperature)	50	200
Highly flammable (supply classification R11)	5000	50 000
Extremely flammable (supply classification R12; flammable and highly flammable substances and preparations maintained at temperatures above their boiling point)	10	50
Substances reacting violently with water (supply classification R14)	100	500

REF 1 Fire Safety Risk Assessment; Factories and Warehouses; HM Government; 2006; available from the Stationery Office: http://www.tso.co.uk. [back]

REF 2 All European legislation is available online at the Eur-Lex portal: http://europa.eu.int/eur-lex/en/. In this article, I have referenced the source European legislation. This legislation is transposed into national Member State legislation, and may appear under different titles. [back]

REF 3 European Agreement Concerning the International Carriage of Dangerous Good by Road (ADR 2005); see: http://www.unece.org/trans/danger/publi/adr/adr2005/05ContentsE.html [back]

REF 4 See Manual of Tests and Criteria, published by the United Nations; http://www.unece.org/trans/danger/publi/manual/manual_e.html [back]

REF 5 The testing methods for supply classification are given as Annex V of the Dangerous Substances Directive (67/548/EEC), and are available at: http://ecb.jrc.it/testing-methods/ [back]

REF 6 Statutory Instrument 2005 No. 1541; The Regulatory Reform (Fire Safety) Order 2005; http://www.opsi.gov.uk/si/si2005/20051541.htm. [back]

REF 7 Council Directive 98/24/EC of 7 April 1998 on the Protection of the Health and Safety of Workers from the Risks Related to Chemical Agents at Work. [back]

REF 8 Directive 1999/92/EC of the European Parliament and of the Council of 16 December 1999; On Minimum Requirements for Improving the Safety and Health Protection for Workers Potentially at Risk from Explosive Atmospheres. [back]

REF 9 See: Guidelines on the Application of Council Directive 94/9/EC of 23 March 1994 on the Approximation of the Laws of the Member States Concerning Equipment and Protective Systems Intended for Use in Potentially Explosive Atmospheres; July 2005. [back]

REF 10 Council Directive 96/82/EC of 9 December 1996, as amended, on the Control of Major-Accident Hazards Involving Dangerous Substances (Seveso II Directive). [back]

REF 11 Guidance on the Preparation of a Safety Report to Meet the Requirements of Directive 96/82/EC, as amended by Directive 2003/105/EC (Seveso II); Report EUR 22113 EN; European Commission, 2005. [back]

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