The quality of stable air plays a major role in a horse’s health, wellbeing, and the lungs’ ability to absorb oxygen. The poorer the ventilation, the longer viruses, bacteria, and dust particles remain suspended in stable air. Mold spores in particular—mainly originating from forage and bedding—are especially dangerous for a horse’s lungs. In the worst cases, prolonged exposure to poor stable air and mold dust can reduce or destroy the function of the alveoli. In severe cases, this may lead to chronic respiratory disease, known as heaves or equine asthma.
In Finland and other Nordic countries, stable ventilation challenges stem from our climate’s wide temperature and humidity variations. The worst environment for a horse is a hot and humid stable. In extreme cases, horses are kept in stables where temperatures rise close to 20°C and all gaps are sealed to prevent draughts. Draught is not a problem for an animal designed to live outdoors. Stagnant air is. Air inlets can be surprisingly large before they cause harmful draughts. Poor stable air, on the other hand, provides an ideal environment for viruses, dust, mold, and fungal spores. In very poor air conditions, an allergy-prone horse may develop heaves within a single winter.
To safeguard respiratory function, horses should receive a steady supply of fresh air year-round and around the clock. In winter, the stable must not freeze, and cold, damp spring and autumn conditions are often especially challenging for ventilation performance. This is problematic because ventilation should work best precisely in humid weather to prevent indoor humidity from rising too high. During the day, stable doors are often opened, and air quality appears adequate. Too often, however, this is forgotten at night. For this reason, the morning air in the stable provides the best indication of how well ventilation truly functions.
The foundation is oxygen and frequent air exchange
The horse’s oxygen uptake capacity is exceptional in the animal kingdom. Thanks to an efficient respiratory system, the horse is both fast and enduring. Muscle work requires oxygen. Any factor limiting oxygen delivery to muscles directly affects performance. At rest, a horse uses approximately 80,000 litres of air per day. During a 1.5-hour training session, it requires 40,000–50,000 litres of air.
Respiratory health and function are therefore prerequisites for athletic performance. This is why the quality of air a horse breathes in the stable is so important for wellbeing and performance.
By law, minimum ventilation requirements for stables are defined as follows:
“Ventilation in animal housing must ensure that humidity, dust levels, and harmful gas concentrations do not rise to harmful levels. The temperature and lighting in animal housing must be suitable for the horse being kept.”
These minimum standards, however, only define the lowest acceptable level and do not guarantee effective ventilation.
For ventilation to be considered effective, stable air should be exchanged approximately four times per hour in spring and autumn, and up to ten times per hour in summer. The minimum ventilation requirement per horse is 80–100 m³ per hour. If the stable is cramped, ventilation must be correspondingly more powerful. If air exchange is insufficient, hay and bedding dust, manure gases, impurities, and pathogens remain airborne. Moisture from the horses’ breathing also accumulates. When air does not circulate, relative humidity rises too high, creating ideal conditions for viruses—conditions that even a strong immune system cannot overcome. Warm, humid stable air allows viruses to survive for several days. Stagnant moisture also accumulates in building structures, significantly increasing the risk of mold and decay, which further worsens air quality. The combination of heat and humidity guarantees mold and fungal growth and is harmful to equine respiratory health. Recommended maximum stable humidity is 60–65%. Recommended stable temperature is +6 to +10°C, and it should remain at least +3°C even in severe frost.
Natural or mechanical ventilation?
Stable ventilation can be arranged either mechanically or by natural (gravity-based) ventilation. Natural ventilation relies on differences in indoor and outdoor air density and height differences between air inlets and outlets, creating negative pressure. Mechanical ventilation uses exhaust fans to remove air. Regardless of the system used, the most important factor is that there are enough air inlets relative to exhaust outlets. The total inlet area should be 2–3 times larger than the exhaust outlet area.
Unfortunately, natural ventilation rarely works well in stables. It functions best when there is a large temperature difference between indoor and outdoor air—essentially when it is freezing outside. In autumn and spring, when temperature differences are minimal, natural ventilation often struggles. It works best in small stables with a large air volume relative to the number of horses and high ceilings. In larger stables or older, low-ceilinged buildings, mechanical ventilation is almost essential.
Ammonia and carbon dioxide – how to measure?
Equipment is available for measuring ammonia and carbon dioxide levels in stable air. Special detection tubes are used to measure concentrations. Measurements should be taken in the morning and in different parts of the stable, as air exchange may vary between aisles and different types of stalls. Fresh air quality is considered good if carbon dioxide concentration is 0.05%, acceptable at 0.1%, and poor if it exceeds 0.1%. Stable CO₂ levels should remain close to outdoor air levels (approximately 0.04%).
Ammonia is released into the air from manure and urine. High ammonia concentrations can impair the function of the cilia in the lungs and predispose horses to infections and other contaminants. Cilia help clear the airways by moving excess particles and impurities out of the respiratory tract. In a well-managed stable, ammonia should not be detectable by smell. Ammonia levels are influenced by temperature, ventilation, and bedding material, and should not exceed 5 ppm. In cool weather, ammonia is less of a problem, but in warm conditions levels rise quickly, making bedding choice especially important. Peat-bedded stalls often maintain relatively low ammonia levels.
Ventilation checklist:
- Removes air impurities
- Removes excess heat in summer
- Removes excess moisture in winter
The foundation of effective ventilation is a system designed to meet the needs of the number of horses housed, and that is cleaned and maintained regularly. Ventilation should be adjustable according to conditions but must never be completely shut off.
Anyone building a new stable or renovating an old one should ensure that the stable can be washed at least once a year. Pressure washing with a mild detergent is the easiest method, and sufficient time must be allowed for drying. Washing improves air quality by removing built-up grime.
Sources:
Airaksinen S., Heiskanen M-L. (2013): Tallinhoitajan opas, Hevostietokeskus
Honkimaa, V. (2018): Hevosenomistaja magazine 2/2018
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