Prevention

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Primary prevention aims to prevent the onset of disease, often by reducing or eliminating exposure to the agent in the workplace. Secondary prevention aims to detect disease at an early or pre-symptomatic stage for example by health surveillance. Tertiary prevention aims to prevent worsening symptoms by early recognition and early removal from exposure and is considered later under the management of an identified case of occupational asthma. The most effective means of control is to prevent exposure altogether either by substituting the agent with a less harmful material or by engineering and hygiene measures. Respiratory protection has a role in situations where control at source is not feasible. With any reported study of preventive measures, it is difficult to distinguish the relative effect of one measure against another, since they are usually implemented as a broad programme with many components including, for example, exposure reductions, worker education and training and stringent health surveillance

Is the incidence of occupational asthma reduced by controlling exposure?

There is extensive evidence of a direct relationship between occupational asthma and allergen exposure (page 14). Further studies have explored the effect of reducing exposure on the incidence of occupational asthma. That reduced exposure leads to fewer cases of occupational asthma has been demonstrated with acid anhydrides (Drexler 1999, Liss 1993), detergent enzymes (Cathcart 1997, Juniper 1977), isocyanates (Tarlo 1997a), laboratory animals (Botham 1987, Fisher 1998) and latex (Allmers 2002, Levy 1999, Tarlo 2001).

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2++

Reducing airborne exposure reduces the number of workers who become sensitised and who develop occupational asthma.

( Allmers 2002, Botham 1987, Cathcart 1997, Drexler 1999, Fisher 1998, Juniper 1977, Levy 1999, Liss 1993, Tarlo 1997, Tarlo 2001)

Is the incidence of occupational asthma reduced by respiratory protective equipment?

Respiratory protective equipment can only offer protection when it is worn properly, removed safely and either replaced or maintained regularly. Brief periods of respirator removal might permit a transient, yet sufficiently high exposure to sensitise a worker and lead to subsequent development of asthma. Studies in this area are few and small. One observed a significant association between asthma symptoms and even brief removal of respiratory protective equipment (Petsonk 2000). Another study demonstrated that respiratory protection was associated with a reduction in the incidence of newly diagnosed occupational asthma but did not prevent the disease altogether (Grammer 2002b)

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3

The use of respiratory protective equipment reduces the incidence of, but does not completely prevent, occupational asthma.

( Cullen 1996, Grammer 2002, Petsonk 2000)

Do pre-placement examinations prevent occupational asthma?

Pre-placement examinations should be used to establish a baseline for periodic health surveillance rather than to detect and exclude susceptible individuals from high-risk workplaces. Little is known about host susceptibility factors, with the exception of atopy in those exposed predominantly to high-molecular-weight agents. The efficiency of screening out susceptible job applicants depends, in part, on the frequency of the trait in the general population. Risk markers such as atopy, smoking, genetic predisposition and sensitisation to occupational allergens lack sufficient sensitivity and specificity for these to be used to screen out job applicants.

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3

The positive predictive values of screening criteria are too poorly discriminating for screening out potentially susceptible individuals, particularly in the case of atopy where the trait is highly prevalent.

( Cockcroft 1981, Juniper 1984, Newill 1986, Niezborala 1996, Renstrom 1994, Slovak 1981, Venables 1988)

It is noted later (page 28) that the likelihood of improvement or resolution of symptoms of occupational asthma is greater in workers who are removed from exposure completely. By extrapolation, workers who already suffer from occupational asthma are at risk from further exposure to the same causative agent, whether exposure is in the same workplace or elsewhere.

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3

A previous history of asthma is not significantly associated with occupational asthma.

( Cockcroft 1981, Gautrin 2001)