Introduction to Health and Safety at Work

14.4 
Classifi cation of hazardous 
substances and their associated 
health risks
A hazardous substance is one which can cause ill-health 
to people at work. Such substances may include those 
used directly in the work processes (glues and paints), 
those produced by work activities (welding fumes) or 
those which occur naturally (dust). Hazardous substances 
are classifi ed according to the severity and type of hazard 
which they may present to people who may come into 
contact with them. The contact may occur while working 
or transporting the substances or might occur during a 
fi re or accidental spillage. There are several classifi cations 
but here only the fi ve most common will be described.
Irritant
is a non-corrosive substance which can cause 
skin (dermatitic) or lung (bronchial) infl ammation after 
repeated contact. People who react in this way to a par-
ticular substance are 
sensitized or allergic
to that sub-
stance. In most cases, it is likely that the concentration 
of the irritant may be more signifi cant than the expos-
ure time. Many household substances, such as wood 
preservatives, bleaches and glues are irritants. Many 
chemicals used as solvents are also irritants (white spirit, 
toluene and acetone). Formaldehyde and ozone are 
other examples of irritants.
Corrosive
substances are ones which will attack, nor-
mally by burning, living tissue. Usually a strong acid or 
alkali, examples include sulphuric acid and caustic soda. 
Many tough cleaning substances, such as kitchen oven 
cleaners, are corrosives as are many dishwasher crystals.
Harmful
is the most commonly used classifi cation and 
describes a substance which, if swallowed, inhaled or 
penetrates the skin, 
may
pose limited health risks. These 

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