Dangerous substances behave in different ways, depending on their type and properties. Knowing how to recognise them is the first step to controlling them.

This module looks at flammable liquids, flammable gases, combustible dusts, oxidisers and reactive substances, and the properties that make them hazardous. It also explains how they are classified and labelled, and how to use safety data sheets.

With a flammable liquid, it is the vapour given off by the liquid that burns, not the liquid itself.

Most flammable vapours are heavier than air. They can spread along floors, flow into drains and pits, and travel a long way to a source of ignition, then flash back to the liquid.

An open container, a spill or a soaked rag can give off enough vapour to create an explosive atmosphere nearby.

The flash point of a liquid is the lowest temperature at which it gives off enough vapour to ignite.

The lower the flash point, the more dangerous the liquid. Petrol has a flash point far below freezing, so it gives off ignitable vapour even on the coldest day. Diesel has a much higher flash point, so it is harder to ignite at normal temperatures, but heating it, or spraying it as a fine mist, makes it far more dangerous.

Flammable gases, such as liquefied petroleum gas, or LPG, hydrogen, acetylene and natural gas, mix easily with air to form explosive atmospheres.

Some are heavier than air. LPG, for example, sinks and collects in low areas such as basements, pits and drains. Others are lighter than air, such as hydrogen and methane, the main part of natural gas, and rise to collect under ceilings and in roof spaces.

Gases are stored under pressure, or as refrigerated liquids. This adds extra hazards: a damaged cylinder or valve can release gas very quickly, and a cylinder in a fire can burst.

Some gases, even non-flammable ones, can also displace the oxygen in the air and cause asphyxiation in enclosed spaces.

Cylinders must be stored upright and secured, in a well-ventilated area, with valves closed and protected when not in use.

Fine dusts from wood, flour, sugar, grain, metals and plastics can form explosive clouds. Some solids are themselves flammable, and some can heat up on their own until they catch fire.

The finer the dust, and the more it is disturbed, the greater the explosion risk. Dust must be controlled at source, with extraction, and must not be allowed to build up on floors, ledges, beams and machinery.

Oxidising substances, such as some bleaches, peroxides and nitrates, give off oxygen. They make fires burn far more fiercely, and can make materials catch fire that would not normally burn.

Some substances react dangerously with water, with air, or with each other. For example, some metal powders react with water and give off hydrogen. This is why careful storage, and keeping incompatible substances apart, is so important.

Whether a gas or vapour rises or sinks decides where it collects, and so where an explosive atmosphere may form.

• Heavier-than-air gases and vapours flow downhill, and gather in pits, drains, trenches, cellars and basements.
• Lighter-than-air gases rise, and gather under ceilings and in roof spaces.

Knowing this helps decide where ventilation openings and gas detectors are needed, high or low.

Dangerous substances are classified and labelled under the classification, labelling and packaging rules, known as CLP. Labels use standard red-bordered hazard pictograms, with a signal word and hazard statements.

• A flame means flammable.
• A flame over a circle means oxidising.
• An exploding bomb means explosive.
• A gas cylinder means gas under pressure.
• A corrosion symbol can mean corrosive to metals, or to skin.

Every hazardous substance supplied for use at work must come with a safety data sheet, or SDS, from the supplier. It has sixteen sections, setting out the substance’s hazards, safe handling, storage, and emergency measures.

Useful sections include the hazards, first aid, firefighting, accidental release, handling and storage, and physical properties such as the flash point. Always read the safety data sheet before you use a substance, and keep it where people can find it.

• Keep only the smallest practical amount of flammable substances in the workroom, no more than the work in hand needs.
• Store larger quantities in a fire-resisting cabinet, or in a dedicated, well-ventilated store, preferably outside.
• Keep containers closed when not in use, and return them to storage after use.
• Separate flammables from oxidisers and other incompatible substances.
• Keep ignition sources well away from storage areas.