Slips and falls are the major cause of accidents reported to the UK Health and Safety Executive. They account for 35% of non fatal major injuries (e.g. fractures and amputations) and 21% of over-three-day injuries (e.g. sprains, strains and minor fractures). A recent estimate of the costs to the UK of these accidents was £1bn p.a.
The Health & Safety Laboratory have recently proposed a holistic risk assessment-based approach as a way of reducing the stubbornly high incidence of these accidents. One element of that approach is footwear. This study shows an informed choice of footwear in a workplace situation can make a significant contribution. The DIN ramp method has been used to identify footwear with good slip resistance in the laboratory. The slip resistant footwear identified has been used in an industrial environment where it has produced significant safety and cost benefits.
Statistics show that pedestrian slipping is the single most common cause of accidents in the UK workplace resulting in more than 34,000 injuries p.a. (HSE, 2001). Slips and trips are now a priority topic for the Health and Safety Commission (DETR, 2001). Our slip potential model suggests five factors that contribute to these accidents: floors, contamination, footwear, use and environment (Thorpe and Lemon, 2000). This paper considers footwear used in a hazardous industrial setting. We have evaluated the relative effectiveness and the economic impact of an alternative form of new footwear.
The slip resistance of footwear can be determined in the laboratory using the HSL DIN ramp test (see figure 1) (HSL, 2001), a derivative of two German Standard tests (DIN 1992(a), DIN 1992(b)). The test uses potable water as the contaminant applied at 6dm3 min-1 through crop jet sprays to ensure a uniform wetting of the flooring surface. The footwear is prepared and conditioned in a standard, reproducible way. The trained operator walks forwards and backwards in “half steps” starting from level. The inclination of the ramp is increased by the operator in approximately 1o increments and the walking test is repeated, until a slip occurs or the operator feels unsafe. The angle at which this occurs is recorded. The test is repeated twelve times, the highest and lowest values are discounted.
The mean of the ten remaining values is the critical angle. The tangent of the critical angle gives the coefficient of friction of the level floor. A second operator repeats the test, the critical angle obtained by the two operators should agree with + or -2o or the tests are repeated. Data for two typical industrial floorings are presented in Table 1. Three different pieces of footwear were evaluated. The Four-S soled footwear provide a benchmark, the other two are example data from the tests. RUB(S) was the footwear which scored highest in terms of slip resistance, and was used in the workplace trial. Pendulum values determined by the UK Slip Resistance Group method (UKSRG, 2000) using a Four-S slider are provided for comparison (see figure 2)
Table 1: Coefficient of Friction (CoF) data; British Pendulum test & HSL SOP-12 DIN test Table
Cost Benefit Analysis
Cost-benefit analyses compare the implementation and impact costs of interventions to their respective benefits to inform decision-makers, in this case, the health and safety professionals in companies at risk of slip-related accidents, of the full impact of available options. The economic evaluation reported in this paper was undertaken at the Doncaster site of Prosper De Mulder (PDM) whose principal activity is rendering animal products. Data was collected during a footwear trial, from July 2001, to January 2002. Using the available data the economic evaluation describes the implementation costs, impact costs and benefits associated with the use of the new footwear.
The quantitative estimates of benefits are described as the number of slip-related accidents avoided using the trial footwear. More hours were worked by workers not wearing the trial footwear so the comparison of the numbers of accidents recorded in each group was divided by the respective number of worked hours for the alternative forms of footwear.
The major facility and head office is based in Doncaster, South Yorkshire. This site employs approximately 300 people in production, transport, engineering and management functions.
Three forms of footwear are worn at PDM – Wellington boots, safety boots, and rigger boots. The trial footwear is not yet available as a rigger boot, so the trial was based on the effectiveness of Wellington boots and safety boots. The cost of the alternative forms of footwear is a function of the purchase price and the lifetime of the footwear. Data on the lifetime of the new footwear were collected as part of the trial, the lifetime of the standard footwear was informed through discussion with PDM. The data was aggregated to estimate the total costs and benefits for the site.
The implementation costs refer solely to the costs of providing the alternative footwear to employees in four distinct operating areas (Table 2). The estimates for the lifetime of the trial footwear were based on the conservative assumption that it was worn out at the end of the trial. In fact more than half of the footwear was still in use. The cost per employee was estimated by multiplying the number of each type of boot required in the trial period by their respective unit costs.
The total costs for the whole plant for the traditional and the trial footwear was calculated by multiplying and aggregating the cost per employee for each of the four operating areas by the total number of workers employed in each of these areas. The resulting estimates show that providing trial footwear to the total 191 workers in the four operating areas would cost an additional £2,953.
In the USA, slips and falls are the second largest source of unintentional injury each year. Overall in 1997 in the USA falls were the leading external cause of medical attended, non-fatal unintentional injuries with 11.3 million episodes reported at an age-adjusted rate of 43.1 per 1000 persons. Slips and falls were also the leading cause (21%) of unintentional injury emergency department visits.
Occupational slip, trip and fall related injuries – can the contributions of slipperiness be isolated?
Courtney TK et al in Measuring Slipperiness – Human Locomotion and Surface Factors Chapter 2. ISBN 0-415-29828-8
Table 2: Implementation costs for alternative forms of footwear Table S = safety boot; W = Wellington boot; R = rigger boot. * seven months trial period † estimates derived from workers, ‡ calculated from the trial data with conservative assumptions (see text)
The impact costs describe the financial impact of the reported accidents and near misses recorded during the evaluation period. The resources used were obtained from PDM’s accident book which recorded the provision of first aid, the need to attend a local A&E department and any other time required off work. Amendments made to the plant’s operations as a result of the accident were also noted. Costs were attached to each resource by applying wage rates to the time-off work, including overtime payments if applicable. Relevant unit costs were applied to the first-aid consumables. Quotes from contracting companies were obtained for major amendments to work activities (such as new flooring), the costs of other interventions e.g. improved cleaning regimes were estimated by PDM’s staff.
Table 3: Accident description and direct costs (averaged) Table
Accident data revealed that fifteen slip-related accidents occurred within the seven-month trial period, no accidents involved workers wearing trial footwear. The accidents were split into three categories of severity. The minor category describes events that were adequately dealt with by a trained first-aider on site. Moderate describes accidents that caused workers to leave their shift early to visit the local hospital or to go home. RIDDOR reportable describes accidents that led to workers being absent from work for more than three days. The frequency of each category of accident and the estimated cost of the provision of first aid and any time-off work are presented in Table 3. In addition some accidents led to remedial actions being taken. Informed estimates of these costs are given in Table 4.
Table 4: Remedial actions and associated costs Table
Further cost savings will be achieved by reduced Employers Liability (EL) premiums to cover personal injury claims. These savings are likely to overshadow all of the cost impacts described above. The data, (excluding any impact on EL premiums) suggests that, if the trial footwear had been supplied to the full workforce during the seven-month period 15 accidents would have been prevented. The additional cost of providing the trial footwear to the full workforce in this period would have been around £3,000, whilst PDM could have saved over £10,000 in lost work-time, and £6,500 worth of remedial actions may have been avoided.
Table 5: Baseline, worst case and best case costs.
The data represents best estimates of the mean effects of the trial footwear. Some of the uncertainties within those estimates can be addressed by a sensitivity analysis. This considers the lifetime of the trial footwear and the frequency of accidents in the trial period. The trial footwear costs are likely to be overestimated, as many had not worn out within the trial period. If we assume a best case of 1.5 pairs of footwear per year, the cost of supplying the trial footwear to the 191 workers most at risk would be reduced to £4,504, a marginal cost of only £905. The number of accidents involving the traditional footwear can be varied to look at the range of accidents that might occur. The incidence of accidents can be assumed to be Poisson distributed. The 95% confidence interval for the number of accidents experienced in the trial period is 7.4 to 22.6 accidents. Baseline, best and worst case scenarios for combined implementation and impact costs of the new footwear in the 191 relevant workers are given in Table 5. A negative cost is a saving to the company.
Figure 8 – Surface microroughness output trace from Surtronic 3+. Taken from typical pressed ceramic tile. Image
In the 1990’s, slips and falls resulted in more than half a million cases of hospitalisation, and killed over 10,000 Australians. In 1995-96 falls cost the community AUS $3.17 billion – exceeding the costs of injuries caused by motor vehicle accidents (AUS $2.7 billion) and all other causes of accidental injury. “The situation is so serious that we now need industry and government to support an Australian ‘roadworthiness’ standard for footwear,” says Mr. Richard Bowman, senior scientist at CSIRO Sustainable Materials Engineering.
CSIRO Manufacturing & Infrastructure Technology – Press release
To accurately predict the slip resistance of a floor any test must generate and manipulate a “hydrodynamic squeeze film” of the same thickness as that produced by a real pedestrian. where Fv represents the normal force applied to the slider, u the slider velocity (assuming that the fixed surface is stationary), the contaminant viscosity, l and b the dimensions of the moving object. For practical purposes, Kp and Ke are taken as constants. The equation presented shows that, in practical situations, contaminant viscosity ( ) has a very significant affect on film thickness (h).
Only two coefficient of friction test methods have been identified which accurately reproduce such squeeze films; one portable (the Pendulum, Fig. 2), and one laboratorybased (the DIN Ramp, Fig.1)
One appreciation of squeeze film theory reveals the importance of surface microroughness in determining slip resistance. Various different surface roughness parameters can be measured using the instruments below, figures 3, 4 and 5. The parameter currently used by HSL is “Rz”. Image
Prosper de Mulder – Footwear Case Study
Prosper De Mulder (PDM) is a family-owned group of private companies. The business is primarily concerned with animal by-product processing and the production of material for the pet food and edible food industries.
“These boots are Figure 7 – Tony Clark and Andy Smith good, even in very slippery conditions. They are very comfortable, I can wear them all day”…Lol Jones, Shop Steward, Safety Branch Sec
A review of accidents on the Doncaster site over the last 20 years revealed that the biggest single cause of accidents was slips. Cleaning, repairs and maintenance regimes and, more recently, the use of anti-slip covers and floor surfaces were clearly only part of the answer to the problem.
At a Food Safety Forum meeting, Tony Clark, PDM’s Health and Safety Officer, heard about a new item of safety footwear specifically designed to be slip resistant in wet and greasy conditions.
PDM ran a trial of safety boots, shoes and wellingtons over a period of several months. During the trial period there were NO slip accidents involving employees using the new footwear. In the 13 months since the end of the trial the new footwear has gradually replaced the previous standard issue footwear. Today 75% of employees have the new footwear (the remaining 25% use a rigger boot, a style not available in the new footwear). PDM remain slip-accident free for employees issued with the new footwear. Wearers have also commented on fit and comfort, which has resulted in increased confidence, improved productivity and better morale. At the same time the previous culture of compensation claims has been overcome which has produced further cost savings.
Discussion and Conclusion
The evaluation reported in this paper improves the knowledge base on the effectiveness and impact of measures to reduce slip accidents. The information can be used by employers to inform health and safety investment decisions, and the revision of health and safety standards that apply specifically to footwear. Though considerably fewer workers wore the trial footwear than the standard footwear, statistically we would have expected to see accidents in the trial group if there was no difference in the effectiveness of the two forms of footwear.
The cost impact of the accidents experienced by workers wearing the traditional forms of footwear outweighed the additional footwear costs. Moreover, the baseline estimates of cost-effectiveness are likely to be at the low end of the true value, due to the assumptions made about the lifetime of the trial footwear. This is particularly true as the workers chosen for the trial were those who wore their footwear especially hard. It is also worthy of note that the slip resistance of the trial footwear was maintained after very heavy wear. This suggests that the improved slip resistance is more likely to be a property of the soling material rather than the intricate cleating pattern.
This is an area which could be usefully investigated in future work. Ideally, monetary values would be attached to the identified benefits, which can be estimated using various techniques (Glaister and Layard, 1994). To date we have not done this for two main reasons. Firstly, employers are less likely to take account of the monetary values associated with the quality of life effects to workers in their investment decisions, and employers are the main target audience for this paper. Secondly, the estimation of monetary valuations of the benefits of such interventions is a long and expensive process.
Given the cost savings associated with trial footwear estimated from the comparison of the implementation costs and the impact costs alone it did not seem necessary to expend further research efforts on monetarising the benefits. The HSL DIN ramp laboratory test clearly differentiated between various pieces of footwear. It clearly identified the trial footwear to have good slip resistant properties. This was borne out by the site based trials. PDM have the opportunity to make considerable cost savings by introducing the new footwear across their different operations, particularly in terms of downstream insurance costs. The trial footwear has reduced the human costs of accidents and has the added benefit of comfort and acceptability to the wearer.
Health and Safety Commission (2001). HSC/E’s long term strategy for slip and trip accidents. http://www.hse.gov.uk/foi/hsc_meetings/2001/papers/c082.htm) (http://www.hse.gov.uk/lau) DETR (2001). Revitalising health and safety. Strategy statement. (http://www.hse.gov.uk/links/revital.htm) Thorpe, S., Lemon, P. (2000). Pedestrian slipping – a risk assessment based approach, Proceedings of the IEA 2000/HFES 2000 Congress. HSL Sheffield UK. (2001) HSL PPESOP12. Standard operating procedure. Inclined ramp CoF test. DIN 51 130 November 1992 (DIN 1992a). Determination of slip resistance. DIN 51 097 November 1992 (DIN 1992b). Determination of slip resistance. Wet loaded barefoot areas. Guidelines recommended by the UK Slip Resistance Group. June 2000. ISBN 1-85957-227-8. Glaister, S., Layard, R. (1994). Cost-Benefit Analysis, Cambridge University Press, Cambridge.
Published: 10th Apr 2003 in Health and Safety International