Respirable Crystalline Silica Exposure in Western Australian Horse Riding Instructors
Summary of Findings

Why the study?

Silicosis is a debilitating disease caused by exposure to respirable crystalline silica. Having been involved with horses for many years, I was interested in whether sand arenas pose a risk of respirable crystalline silica exposure for workers spending a lot of time working in them, as I am familiar with how dusty they can get. I also wanted to assess whether the simple control of irrigation would be effective in reducing silica exposure.

What did the Study Determine?

The results of the study clearly demonstrate that horse riding instructors teaching students in a non-irrigated, sand based horse riding arena are very likely to be exposed to respirable crystalline silica levels above the occupational exposure standard. On a scale of low to critical, the relative health risk associated with teaching full time (40hrs/week) in a non-irrigated sand arena has been determined to be critical.

Conversely, horse riding instructors teaching students in an irrigated sand arena are unlikely to be exposed to respirable crystalline silica levels above the relevant occupational exposure standard, regardless of whether the surface is sand or sand composite. The relative health risk associated with teaching full time (40hrs/week) in an irrigated sand arena has been determined to be moderate. As such, this study found that irrigation is an effective control to reduce the exposure of horse riding instructors to respirable crystalline silica.

Figure 1: Graph showing the number of samples in the study having results in each concentration band. Bars are colour coded for irrigation status and arena surface type. (Click for full size)

The self-reported incidence of respiratory health effects (coughing, wheezing, dry throat, sneezing or shortness of breath) was found to be significantly higher (71% as compared to 37.5%) in the group of horse riding instructors with measured respirable crystalline silica exposure results >50% of the occupational exposure standard concentration, than in the group with measured exposure results <50% of the occupational exposure standard concentration. These results indicate that instructors with higher exposures to respirable crystalline silica have a higher incidence of respiratory symptoms.

What is so bad about respirable crystalline silica?

Respirable dust comprises all of the very small particles (≤10μm in diameter) in the dust cloud. Due to their small size, these particles are able to penetrate into the deepest regions of the lungs where they are less able to be removed by the body. Respirable dust may contribute to Chronic Obstructive Pulmonary Disease (COPD).

Respirable crystalline silica is a highly toxic component of the respirable dust cloud. Overexposure to respirable crystalline silica causes silicosis. Silicosis causes scarring of lung tissue and debilitating shortness of breath. This can be severe and may be progressive and worsen even when exposure stops. In addition, silicosis is associated with an increased incidence of lung cancer.

Respirable crystalline silica exposure typically causes health effects following 10 to 30 years of elevated exposure.

What is the exposure standard?

The exposure standard is the atmospheric concentration at which the 'average' person should not experience significant negative health effects after exposure for a 40hr working week, for a working lifetime. It should be noted that the exposure standard is a guideline concentration that is not protective for all persons. Action should be taken to reduce any exposure above 50% of the exposure standard.

As a general rule, exposure to respirable crystalline silica should be kept as low as is reasonably practicable.

I dont teach horse riding for 40 hours a week, is my exposure still high?

Figure 2: Graph showing the weekly exposure time before which the exposure standard is exceeded at given levels

As many instructors do not teach horse riding for 40 hours a week, their actual exposure time is often significantly lower than that on which the exposure standard is based. It can be useful to consider exposure in terms of 'dose' where exposure for 40hrs/week at the exposure standard concentration is considered a 'full dose'. So exposure for 20hrs/week at the exposure standard concentration, or an equivalent combination, would be a '50% dose'. Exposures above 50% require action to reduce silica exposure. Further detail is shown in the graph below.

It should be noted that whilst a 50% exposure dose might be 'acceptable', it is by no means ideal. Every effort should be made to reduce the amount of dust, rather than relying or a reduction in exposure time.

Is there any recorded incidence of silicosis occurring in horse riding instructors?

I was unable to find any recorded incidence of silicosis in horse riding instructors, however an incidence of lung cancer in a Korean race horse trainer was attributed to overexposure to respirable crystalline silica. During the training he routinely lunged the horses in a semi-enclosed, non-irrigated sand round yard.

Numerous instances of silicosis have been found in farm workers who were working in sandy soil areas. This indicates that silicosis is not limited to working environments where silica particles are actively being fractured (such as during drilling or blasting processes) and that disturbance of sand can also cause silicosis.

Could my horse get silicosis?

It is possible. Some cases of equine silicosis have been reported and exposure has thus far been attributed to sandy living conditions. The exposure level required to cause disease in a horse is unknown, however the low reported incidence of silicosis in horses indicates that exposure sufficient to cause disease is unlikely.

It should be remembered that the actual exposure of any horse is dependent on both the concentration of the respirable crystalline silica and the duration of exposure.

How can I reduce my exposure?

A number of options are available to reduce exposure to respirable crystalline silica from horse riding arenas. Not all are feasible in every situation and most involve compromises in some way.

This study found that irrigation of the arena surface prior to use is an effective control for dust. The other controls listed are options to consider which may be effective.

1. Use an arena surface that does not contain sand. Some alternative surfaces to sand are:
   • Grass / turf (irrigation will probably be required, so it might be easier to irrigate your sand arena)
   • 100% Wood chip (to control silica exposure, however organic dust generation is likely unless irrigated)
   • 100% Shredded rubber (may pose a heat stress risk in summer if black rubber is used)
   • Combination of wood chip, rubber or similar product that does not contain sand.
2. Treatment of the sand surface to reduce dust. Some options are:
   • Use of Wax coated sand products
   • Use of Polymer coated sand products
   • Use of wax or oil additives on existing sand arenas
   • Use of salt additive (will work in humid environments only)
3. Irrigation of sand surfaces prior to teaching has been found in this study to be an effective control measure. To minimise incidences of lameness, the FEI recommends that sand arena moisture content is maintained at 8-17% moisture. It is therefore important for both water conservation and horse injury prevention, that the surface moisture level is controlled and excess water not applied. Consider:
   • Permanent irrigation setup
   • Use of a water truck to irrigate grounds prior to an event. (May be a more feasible option for council owned or shared facilities that are used irregularly, or where water availability is problematic).
4. Administrative controls involve modifying your behaviour to control exposure, some examples include:
   • Reduce the exposure time by restricting the hours spent teaching or otherwise working in the arena during dry conditions (to remain under the '50% dose').
   • Use alternative venues during dry conditions if dust at your normal venue is unable to be effectively controlled.
   • Wherever possible stand upwind or some distance from the horses when teaching.
   • Raise awareness of the potential for health effects from working on dusty sand arenas, by sharing this information with other horse riding instructors or people in the process of constructing arenas.
5. Respiratory protection is not considered appropriate during teaching, but is recommended as an appropriate and effective control for exposures during any arena maintenance activities. A correctly fitting 'P2' respirator is required to control exposure. Men wearing respirators should be clean-shaven to ensure a close fit.

What were the limitations of the study?

Due to limited numbers of volunteers and arenas available for testing that were sand composite material but not irrigated, evaluation of the addition of other materials such as rubber or bark to the sand was unable to be effectively evaluated as a control measure for respirable crystalline silica exposure.

The speed of horses travelling in the arena and the distance separating the instructor from the horses was not able to be assessed in this study. It is considered likely that these factors will influence the results.

The results are representative of exposures on the day of monitoring only. They have been considered as representative of average exposures for the purposes of this study, but actual exposures are likely to vary significantly with environmental conditions, horse activity levels and the distance between the horse and the instructor during the monitoring.

If you have any questions about the study, please don't hesitate to contact me by email at sarajackson@live.com.au.