Occupational Exposures in an Equestrian Centre to Respirable Dust and Respirable Crystalline Silica

ABSTRACT

Sand-based products are regularly used as footing material on indoor equestrian arenas, creating a potential occupational exposure risk for respirable crystalline silica (RCS) for equestrian workers training and exercising horses in these environments. The objective of this study was to evaluate an equestrian worker’s personal RCS and respirable dust (RD) exposure. Sixteen personal full-shift RD measurements were collected from an equestrian worker and analysed for RD, quartz and cristobalite. Geometric mean exposures of 0.12 mg m−3 and 0.02 mg m−3 were calculated for RD and RCS concentrations, respectively. RCS exposures of between 0.01 to 0.09 mg m−3 were measured on days when the indoor arena surface was not watered, compared to lower exposures (<LOD-0.03 mg m−3) on days when the indoor arena was watered (p < 0.01); however, manual watering is time intensive and less likely to be implemented in practice. This small-scale study provides new data on RCS and RD exposures among equestrian workers. RCS exposures are within the range considered to be associated with increased risk for lung cancer. The use of dust control solutions such as water suppression should be promoted for equestrian work in horse riding arenas. Equestrian workers need to receive occupational health training on the health risks associated with RCS exposure.

MATERIALS AND METHODS

One small to medium sized Irish equestrian centre, managed and operated by a self-employed worker was recruited to participate in the study, over the summer period of 2018. The centre stabled, on average, 30 horses for training and a further 15 horses for riding lessons, had one indoor arena (approximate area of 6000 m2) and two outdoor arenas (each had an approximate area of 10,000 m2), all surfaced using silica sand and shredded carpet mix (which is sand mixed with polypropylene, polyester and polyurethane fibres shredded into pieces <30 mm in length). The indoor arena was housed under the same roof as the tacking and grooming area, it had entrance sliding doors on two side walls, which were closed during the surveys. There was no mechanical ventilation. Horses were brought into the arena via the tacking and grooming area. The building also contained a small fully enclosed room under the same roof, which functioned as a canteen. The room had windows, which opened onto the arena area; however, they were rarely opened. To reduce dust levels in the indoor arena, occasionally, the surface was dampened using a water hose mounted on a ladder (approximately 3 m high), which was moved around the arena for a maximum of one hour. However, this water suppression regime was rarely performed and only if there were no competing work tasks to be performed at the centre. A convenience sampling approach was followed when collecting personal exposure data and measurements were collected when the worker performed their normal daily duties. The worker was sampled at standing height (1.5 m), apart from when they were grading/raking when then were at sitting height. Typical work duties included;

- cleaning horse stables,
- longeing horses (which involves the horse, attached to a lunge line, moving around the trainer),
- loose/free jumping 2–4 young horses per day (involves jumping a horse without a rider to practice the horses jumping skills),
- delivering riding lessons both indoors and outdoors,
- grooming, tacking and untacking horses which was always performed in the indoor arena,
- grading/raking the surface of both indoor and outdoor arenas. Surface grading is required to maintain a good workable footing material, which over time, becomes compacted due to horse traffic. In this study, the surface was graded using a rake attached to an open top tractor which was driven around the arena.

Working with the horses when longeing, free jumping or during riding lessons and raking the arena surface lead to increased dispersion of the footing material and visible clouds of dust in the arena. Personal breathing zone samples were collected using a personal sampling pump (Sidekick; SKC Ltd., Dorset, UK) with Higgins-Dewell cyclone (Casella, Bedford, UK) and 25 mm, 5 µm pore size PVC filters. Pumps were pre-calibrated at a flow rate of 2.2 L per minute (L min−1) using a primary airflow meter (DryCal® DC Lite; BIOS International, Butler, NJ, USA). The researcher collected contextual information to support all samples collected, including time spent on potentially high-risk exposure work tasks (tasks which generated visible clouds of dust in the work area). RD samples were collected and analysed gravimetrically according to HSE MDHS 14/4, the limit of detection (LOD) for RD was 0.05 mg. Both quartz and cristobalite was quantified on each sample using X ray diffraction using a Bruker D2 Phaser X ray Diffractometer with Bruker Diffrac. DQuant 1 software following HSE MDHS 101/2. Sample analysis was performed at the Institute of Occupational Medicine, Edinburgh who are accredited for XRD analysis by the United Kingdom Accreditation Service (UKAS).

Summary statistics were calculated using SPSS version 26, a concentration data was log normally distributed, a paired t-test was used to compare RD and RCS exposures on days when the indoor arena was watered to when it was not watered. The results were compared with the Irish Occupational Exposure limit value for RCS, 0.1 mg m−3 and the recommended comparison guideline for low-toxicity respirable dust, 1.0 mg m−3. Where sampling times exceeded 8 h, exposure concentrations were adjusted to an 8 h reference period. Estimates of relative risk for lung cancer were calculated using RCS exposure data and log linear response curves derived by Steenland et al.

RESULTS

A total of 16 personal exposure measurements were collected from the one equestrian worker over the period of June–August 2018. The worker was sampled for the full work shift, including short break periods which were always spent in the arena canteen, lunch breaks (30 min) were spent off-site and not included in the measurement. Sampling times ranged from 480–540 min and the worker spent between 75% and 85% of their time working in the indoor arena or nearby during the measurement period.

Exposure Results

Individual personal RD and RCS concentrations (mg m−3) are presented in Table 1. Table 1 also provides a summary of the work activities undertaken during each of the measurement periods, whether water suppression was applied in the indoor arena and also, outdoor weather conditions. The outdoor weather conditions were dry on 14 of the 16 days surveyed; there were light rain showers on days 8 and 9. On four of the measurement days, the surface of the indoor arena was sprayed with water in the morning, and on another four days, it was sprayed in the morning and afternoon.

Cristobalite was not detected in any sample and so the RCS results reflect quartz exposure. Two of the sixteen personal samples had non-detectable levels of RD and RCS (samples 15 and 16). Sample geometric mean (GM) and geometric standard deviations (GSD) were calculated by substituting < LOD values with half the analytical LOD, this being 0.025 mg for RD and 0.005 mg for RCS. GM (GSDs) of 0.124 mg m−3 (2.15) and 0.025 mg m−3 (2.43) were calculated for RD and RCS concentrations, respectively (range; <0.05 to 0.30 mg m−3 (RD) and <0.01 to 0.08 mg m−3 (RCS).

There was a strong positive correlation between RD and RCS concentrations (p < 0.05). There was a significant difference (p < 0.01) between concentrations of both RCS and RD exposures on days when the indoor arena was watered and on days when no watering was performed.
