To examine the effects of installing a mechanical ventilation system at a riding-school stable on indoor air quality and human and horse airways.

The intervention was the installation of mechanical ventilation in a riding-school stable. Carbon dioxide (CO₂), ammonia, particles, horse allergen, microorganisms and endotoxins were measured in the stable. The stable-workers and riding-students completed a questionnaire and underwent the following tests: analysis of nasal lavage for inflammation biomarkers; levels of exhaled nitrogen oxide (NO); measurements of daily peak-expiratory flow (PEF). The horses were examined clinically by airway endoscopy and bronchoalveolar lavage (BAL) and were analysed for cytology and biomarkers.

Levels of CO₂ were nearly halved and airborne horse allergen levels were markedly reduced (5–0.8 kU/m³) after the intervention. A decreased level of ultrafine particles was observed (8000–5400 particles/cm³) after the intervention, while total and respirable dust levels were mainly unchanged (200 and 130 μg/m³). Levels of microorganisms in surface samples decreased following the intervention, whereas airborne microorganisms and endotoxin increased. There was no significant change in human symptoms, PEF-variability, exhaled NO or inflammatory biomarkers in the nasal lavage. In horses, the mean score of lower airway mucus was significantly reduced together with the mean level of expression of interleukin-6 mRNA in BAL cells after the intervention.

The installation of a mechanical ventilation system resulted in an increased air exchange rate, as demonstrated by reduced levels of CO₂, ammonia, ultrafine particles and horse allergen. There was no significant clinical effect on human airways, but there was a tendency for reduced inflammation markers. The results on the horses may indicate less impact on their airways after the intervention.