Your horse will inhale and exhale about 4 litres of air in each breath.

When exercising hard, that volume will increase enormously. So in order to perform to their full potential, your horse's lungs must be clear and functioning properly.

If your horse inhales dust from the bedding you have put down in the stable, there is a good chance that this dust will cause a reaction in the soft membranes that line the respiratory passages (the nose and windpipe) and in the lungs themselves.

At the very least, this will result in irritation of the membranes, but it might cause a more severe allergic reaction. Depending on the individual horse and the degree of reaction, the membranes will take steps to repel the invading dust. Inflammatory cells will gather at the site, the muscles of the lungs will contract, and extra mucus will be produced to surround the irritant. This mucus will be stickier than usual and therefore more difficult to dislodge, resulting in coughing and possibly in a discharge from your horse's nose. Lungs and airways will become constricted, breathing more difficult and lung capacity reduced. Your horse will feel just like you do when you have a bad cold.

If the reaction is mild you may not notice any obvious symptoms - just a feeling that your horse is off colour, or doesn't perform quite as well as you expect. In some respects this is worse. You know there is a problem, but you might not guess the cause.