The quality of the air in the pig building depends on a number of factors including the stocking density, the cubic capacity of the building, the lower critical and upper critical temperatures, concentrations of gases and levels of dust.

As the number of infectious organisms and the pigs' exposure to them increases, there is an increasing risk of disease. Control of the environment therefore must constantly aim to reduce these levels.

Bacteria and viruses spread from pig to pig by direct contact, indirect contact (e.g. on walls or floors), on equipment and people, and by airborne dust and droplets. The latter mode of spread is obviously associated with air quality.

In dry airborne dust most of the infectious organism die quickly but their toxins (e.g. endotoxins) can still be harmful to the pig when inhaled. Aerosol droplets containing organisms dry out rapidly at low humidity and the organisms die. At the middle range of humidities the droplets do not dry and the organisms remain viable and infective. At very high humidities (>90%) droplets and dust pick up water, increase in size, and are precipitated out of the air.

Size of the dust particles or aerosol droplets has a bearing on the pigs' defences. When very small particles or droplets are inhaled they are sucked deep into the lungs, sometimes as far as the terminal air sacs (alveoli). Larger particles and droplets tend to be filtered out in the nose and throat or in the upper airways of the lungs.

Using systems that do not operate on an all-in all-out basis the incoming pigs meet with heavy doses of respiratory and enteric organisms which are being shed by the older pigs already there. The most harmful time for this to happen is in first stage and second stage weaner accommodation at a time when the antibodies from the sows colostrum and milk are waning and before the pig has had time to fully develop its own. Overwhelming aerosal build-up of organisms also tend to occupy finishing rooms that contain heavier pigs.

The constant high challenge to the respiratory system results in major mobilisation of the pigs immune systems which is costly in energy and lowers the pigs growth rate.

The situation may be worsened by suppression of the pigs' immune system caused by mixing, moving, stress, overcrowding, inadequate nutrition, and high levels of irritant slurry gases. Increased ventilation helps in reducing gases but does not have much effect on airborne dust and droplets. Infact it may make airborne dust levels worse. It also may create evaporative cooling over the pigs which will not help their resistance. Chilling tends to trigger disease.

Reducing stocking density and avoiding overcrowding have the biggest effect on improving air quality and reducing airborne organisms with a resultant boost to growth rate and efficiency of feed conversion to meat. This increased growth also serves to decrease the stocking density by faster throughput. Changing from dry feeding to liquid feeding also reduces dust levels and respiratory disease.

A checklist for maintaining air quality

• Assess the quality at the daily inspection.
• Humidity.
• Condensation.
• Smell. Levels of ammonia.
• Dust levels.
• Assess the lying patterns of the pigs. Are there draughts?
• Check temperature fluctuations.
• Check that fans and inlets are functioning.
• Strip and clean the inlets and outlets between batches of pigs.
• Test that controller systems are functioning correctly each week.
• Test the fail safe mechanism twice weekly.
• Assess levels of disease.
• Check the stocking density of the pigs relative to the cubic capacity of the house.
• Check that stocking densities are not above recommended levels.