Did you know that...The dust which you cannot see in the air is the most dangerous

Large-size dust particles greater than 5 microns (μm) in diameter, such as contained in dry sawdust, wood shavings and rice hulls used as stable bedding – the dust you can see drifting and swirling around in the air with the naked eye, settles out and adheres to the moist nasal passages when inhaled.
However, smaller dust particles less than 5 microns (μm) in diameter, which include moulds and airway pollutants, as well as irritant ammonia gas released from urine, droppings, decaying straw and bedding materials, are taken up in the airflow and inhaled deep into the lungs.
Even sawdust and wood shavings can be virtually ‘ground’ into smaller airborne particles as a horse with metal shoes walks and moves around in a stable.
The ‘grinding’ effect of the shoes also occurs in dry sand yards and lunging arenas,
which eventually reduces particle size of coarse sand, making the small particles,
including decaying droppings mixed into the surface, more likely to be inhaled as a
horse walks around ‘hoovering’ the surface for spilt food morsels, or exercises in a
dry, dusty yard or an arena.
This is why it is important to ensure that a horse is able to put its head down for at
least 20-30 minutes each day when eating hay, which should be dampened and fed
in a feed bin (eg tyre feeder) on the stable floor or a sheltered area of an outside
yard.
Turning a horse out to graze for 2–3 hours after each day’s exercise also
helps to naturally drain away inhaled dust and irritant particles.
If a horse has little opportunity to eat with its head down to drain its lower airways, it has an increased chance of developing chronic low grade airway disease.
If it becomes infected with the ‘stable virus’, although normally confined to the nasal passages, throat and upper airways, without adequate head-down drainage, the inhaled mucus and infective viral particles have a risk of establishing chronic lower airway disease.

Article courtesy of Dr John Kohkne BVSc RDA - Talking Horses newsletter