Background Organic dust from hay is a primary trigger of equine asthma.Objectives (1) To introduce a novel hay-shaker (HS) device for standardised hay dust generation, enabling simultaneous measurements of various particulate matter (PM) size fractions. (2) To compare these measurements with those in the horse's breathing zone (BZ) to assess the influence of environmental and individual factors.Study Design Comparative experimental study.Methods A HS generating dust was developed. Total dust (PMT) and size fractions (PM1, PM2.5, PM4 and PM10, representing particle size in μm) were measured from 50 hay samples provided by clients using an aerosol monitor (DustTrak DRX 8534) in the HS (HS-PMx) and the BZ (BZ-PMx) of 50 horses (26 healthy, 24 asthmatic) during hay feeding. Linear regression analysis evaluated the relationship between HS-PMx and ln BZ-PMx for each fraction, accounting for environmental (humidity, temperature) and individual factors (horse characteristics and feeding behaviour, assessed with the Hay Contact Score).Results The HS generated measurable dust across all PM sizes. Regression models explained 69% to 73% of ln BZ-PMx variance, showing a weak positive association between HS-PMx and ln BZ-PMx across all size fractions. Hay Contact Score had the strongest positive association with ln BZ-PMx. Cohort, ambient temperature and humidity were negatively associated with ln BZ-PMx for certain particle sizes. The final model, incorporating HS-PM4, Hay Contact Score, cohort, temperature, and humidity as predictors, demonstrated robust predictive accuracy for BZ-PM4 (adjusted R2 = 0.73).Main Limitations Clinical impact of hay dust and poor-quality hay was not assessed.Conclusions The HS reliably generates hay dust for measuring standard PM fractions, particularly respirable PM4, critical to equine respiratory health. BZ dust concentrations are significantly influenced by feeding behaviour. The HS offers a standardised method for assessing hay quality, enabling informed decisions on hay selection to support respiratory health in stabled horses.
