8.4 Lifting and Handling
8.4.3 Weight limits
What is the maximum weight that a person of a particular age and sex, and of ‘normal fitness’, may be expected to lift under a given set of circumstances, without undue risk of injury?
This is a very difficult question indeed. First of all, there is no weight of load, however small, that guarantees safety.
You can injure your back by stooping down to pick up a pencil. Biomechanical calculations show that when the trunk is inclined forward to a horizontal position, the loading on the base of the spine is the same as the loading that results from lifting a compact 30 kg weight close to the body (Pheasant 1991a). So if for one reason or another the back is at all vulnerable to injury there may be no spare capacity for actually lifting an external load, over and above the weight of the body itself. Unfortunately, the precursors of lifting injury, which determine a person’s level of vulnerability, are not always easy to recognize. A person may be vulnerable to injury without knowing it. Overall we are not particularly good judges of what we can handle safely—and we commonly injure ourselves lifting loads that we believe to be within our capacity. Conversely, experience shows that if a load feels as if it is too heavy to handle safely then it very probably is too heavy. In other words, our subjective appraisals of our safe limits are systematically biased in the direction of risk (although there are doubtless important individual differences in this respect).
In any given set of circumstances (as defined by load characteristics, lifting position, and so on), we should in general expect the risk of injury due to over- exertion, for a particular individual, to increase steadily with the weight of the load.
The relationship may or may not be linear (but biological systems being what they are, a non-linear relationship seems the more probable). Given the nature of human variability, then the rate at which the risk of injury increases with the weight of the load will necessarily vary greatly between individuals. So if there are important threshold effects (where the level of risk takes a sharp upswing) then the location of these will likewise vary greatly. Taking one consideration with another, one would expect the risk of injury by over-exertion, for a given working population, to increase with load weight in a smooth upwardly accelerating curve, in which such threshold effects as might be present for given individuals are masked.
We also need to take into account both the immediate risks of over-exertion injury and the long-term risks of cumulative over-use—and probably also the interactions between the two. The risk of accidental injury presents an even more difficult set of problems. One would expect the probability of some unexpected mischance that leads to injury to increase with the weight of the load; but the connection is not as clear as it is for over-exertion injury or cumulative over-use, except insomuch as the heavier the load, the more likely you are to injure yourself seriously if it gets out of control.
Even if we fully understood these matters, to the extent of being able to plot out a curve relating overall population risk to load weight, we should still be faced with the question of where to set the limit. What is the cut-off point beyond which the risk of injury becomes unacceptable?
One possibility is to set the limit at a level of loading that would result in a just noticeable risk of injury: that is, at which the risk of injury due to work would be just measurably greater than the background level of risk associated with life as a whole.
This is, broadly speaking, what we attempt to do for chemical hazards, radiation hazards, and so on, since in these cases the risk in question may be controlled by a
more effective containment of the hazard so that the working person does not come into contact with it. In the case of lifting and handling, however, we cannot do this—
and to set the limit at the level of just noticeable risk would to all intents and purposes be equivalent to calling for the abolition of all useful manual work. This would be pointless—not least because guidelines that cannot be met in practice are ignored and fall into disrepute.
Neither does it make much sense to set the limit at a level of loading at which injury becomes ‘probable’ to the extent of being ‘more likely than not’, because in practice this would mean that we were constantly having to replace our workforce. There are industries where this happens. They are recognizable by the age distributions of their workforces. The ambulance service is a notable case in point.
The problem thus becomes one of reaching a reasonably practicable compromise position which allows working life to continue without incurring excessive risk. This roughly approximates to the legal concept of a risk that is reasonably foreseeable; or, in this context, the level of risk which a ‘reasonable person’ would agree to accept in the course of his or her working life (were he or she fully appraised of the facts of the matter).
In reaching this point we are faced with two principal difficulties: the first is the inadequacy of our scientific knowledge; the second is that legal conceptions of probability are not altogether the same as scientific ones, in that ‘moral certainty’
does not equate easily with statistical certainty.
Some countries in the world have seen fit to impose limits on the weights that people may lift at work; others have not. The International Labour Office has published a compilation of such weight limits (ILO 1990). These are summarized in the form of a cumulative distribution in Figure 8.11. The graphs show the percentage of countries in which a load exceeding the weight in question would be considered unacceptable. For adult men these cluster around a median figure of 50 kg, with 50%
of values falling within the 45–55 kg range; for women they cluster around a median value of 25 kg, with 50% falling within the 20–25 kg range.
The US National Institute of Occupational Safety and Health has published an influential set of guidelines which deal specifically with symmetrical two-handed lifting actions performed directly in front of the body (NIOSH 1981). For any such action it is possible to calculate an action limit (AL), beyond which there is deemed to be a moderate increase in risk; and a maximum permissible limit (MPL), beyond which the risk is considered unacceptable. These guidelines are based upon bio- mechanical, physiological, psychophysical and epidemiological criteria. The equation that defines the AL and MPL takes into account the horizontal and vertical positions of the load, the distance it is lifted, the frequency of lift, and the duration of the task. The original guidelines have subsequently been revised (Waters et al.
1993). The equation defining the limits has been modified, and two new elements, dealing with asymmetry and ease of grasp, have been added. The action limit and maximum permissible limit have been dropped in favour of a recommended weight limit (RWL) which is set at a level that is approximately equivalent to the old action limit; and the concept of a lifting index (LI) has been introduced. This is the ratio of the load on the job to the RWL. It thus represents a relative measure of the severity of risk. It is essentially seen as a tool in job redesign and no specific cut-off point is proposed. I have discussed the theoretical and practical strengths and weaknesses of the NIOSH guidelines at length elsewhere (Pheasant 1991a). I would further add
that in my view both the old and new NIOSH equations greatly underestimate the importance of the vertical height range of the lift (see above).
The set of guidelines that was devised at the Robens Institute of Health and Safety at the University of Surrey was based upon an extensive series of laboratory experiments and field studies in which intra-abdominal pressure (IAP) was used as an indirect index of spinal loading. On the basis of the field studies an IAP of 90 mm Hg was adopted as the safe limit for men (Davis and Stubbs 1977, 1978); and an equivalent level of 45 mm Hg was subsequently adopted for women (David 1987).
The Robens Institute figures have been incorporated into a UK Ministry of Defence Standard (Ministry of Defence 1984). The guidelines are presented in the form of contour maps, drawn in elevation and plane—examples of which are shown in Figure 8.12. The contour maps represent the level of loading at which the IAP criterion will be violated on only 5% of occasions. Similar maps are provided for pushing and pulling actions and correction factors are given for age and sex.
The ‘guideline figures’ published by the UK Health and Safety Executive (HSE 1992), and shown in Figure 8.13, are based in part upon the Robens Institute load limits and in part upon the concept of lifting zones as set out above. The document in which these are set out stresses that they are not ‘limits’ as such, but are for guidance purposes only—to be used in the context of a broader approach to the assessment of risk based upon ergonomic principles. The guideline figures are said to be such as to afford ‘reasonable protection’ to nearly all (95%) of men and between one-half and two-thirds of women. A correction factor of one-third is sug-
Figure 8.11 Statutory limits on the weight to be handled by one worker in different countries of the world. Cumulative distributions are based on data given in ILO (1990). (From S.Pheasant, Ergonomics, Work and Health, Macmillan, 1991, fig. 15.26, p. 314, reproduced with kind permission.)
various positions in the zone of convenient reach (ZCR) as given by Davis and Stubbs (1977, 1978) and Ministry of Defence (1984). Values given are for men under 50 lifting less than once per minute.
Figure 8.13 Lifting zones with the HSE guidelines data (measurements in kg).
Figure 8.12 Suggested limits for lifting forces (kgf) two-handed (left) and one-handed (right) at
gested as giving the same degree of protection to nearly all women. Correction factors are also given for twisting, repetitions rates and team lifting.
In essence, the guideline figures define a boundary level beyond which there is a potential risk of injury. The HSE document in question goes on to say that operations exceeding the guideline figure by a factor of more than about 2 should ‘come under very close scrutiny’—presumably in terms of risk assessment. I take this to be a way of saying that beyond this point the risk of injury may well be unacceptably high.