Efficient cooling methods to reduce heat stress in pigs
Microclimate parameters such as air temperature and humidity are important indicators of animal welfare. Besides, heat stress is more detrimental to productivity in the swine industry than swine diseases. Several cooling methods are effective in reducing heat stress in pigs while being energy efficient.
Indicators of animal welfare include variations in parameters related to animal physiology, behaviour, and productivity as well as microclimate parameters such as temperature and humidity. The increase in the frequency of heat waves observed in recent decades has increased the vulnerability to heat stress among livestock in indoor facilities. In pigs, heat stress triggers behavioural and physiological response. One of the most important reactions to heat is a 30-50% reduction in voluntary feed intake at temperatures around 30°C and above.  Reduced food intake greatly affects productivity. In the US for instance, the economic impact of heat stress on the livestock industry has been estimated in billions of US dollars.  Due to the consequences of heat stress, US hog producers lose approximately US$900 million each year, twice as much as losses caused by swine diseases. 
Research suggests that thresholds for indoor climate parameters to avoid heat stress in pigs include an air temperature below 25°C and a Temperature-Humidity Index below 75.  Some of the most common methods of cooling pigs are listed below. 
Cooling Methods Based on Water Evaporation
Evaporative cooling is based on the principle that the evaporation of a liquid absorbs significantly more heat than the amount of heat required for the liquid temperature to rise by a few degrees. 
- Direct evaporative cooling
The direct evaporative cooling is as follows: outdoor air passes through an enclosed space where it comes into contact with water, either in the form of fine droplets or by saturating a porous medium. The water evaporates into the air, thereby making it cooler and moister. 
In livestock confinement buildings, the following methods are used: fogging, misting or showers as well as drop cooling. With the fogging or misting methods, water droplets are sprayed on a small or larger surface. In the case of drop cooling, nozzles of the system are often installed in individual pens above the animal’s neck. Since pigs are non-sweating animals, these methods first moisten the animal skin and then cause the water to evaporate from the surface of the animal body, thereby improving evaporative heat loss. 
Associated with a fan system, cooling pads or evaporative pads are designed to cool the air before it gets into the building. A fan forces the flow of warm air through water-soaked material, usually cellulose paper, shaped in a way that allows molecules of water to mix with air. Evaporative pads may have large or small surfaces. The smaller surface evaporative pads are often used to cool air conducted through plastic ducts with holes or inlets above the animal’s head (snout cooling). Larger pads are often used with tunnel ventilation.
The literature reports that direct evaporative cooling methods have a positive impact on food intake as well as on the regulation of skin temperature and respiration rate. The drawback of direct evaporative methods may be an increase in relative air humidity, which must then be mitigated by the installation of desiccant segments. Water conservation is also a major concern. Studies indicate that evaporative pads are a better solution forwater savings than fogging systems or direct sprinkling of animal skin. 
- Indirect evaporative cooling: cooling pads combined with a heat exchanger
The air used to cool the room passes through a heat exchanger placed in an enclosed evaporatively cooled space. This system results in a temperature drop comparable to direct evaporative cooling, but without humidification, since the amount of water vapour in the air remains unchanged. The presence of the heat exchanger results in a slightly lower temperature drop than in direct cooling systems. [2,4]
Technologies Based on High Forced-Air Velocity
Forced convection is one of the most effective ways to improve an animal’s heat loss. These systems can be implemented either to cool animals in a special zone (e.g. snout cooling) or with high air speed over the animals in the whole living area (e.g. through tunnel ventilation). Tunnel ventilation at an air speed of about 2 m/s is a popular solution.
The cooling effect achieved by convective heat transfer may be further enhanced by evaporative cooling methods. However, a study found no significant differences in production indicators when comparing pigs cooled by tunnel ventilation alone versus tunnel ventilation associated with evaporative cooling. 
Technologies Based on heat exchangers
More energy efficient systems using heat exchangers have been developed.  For instance, an earth-air heat exchanger uses the ground as heat storage, with outside air flowing through tubes buried at 1 to 3 m deep. Such a system achieves better cooling performance than evaporative cooling methods or ventilation alone methods.  An earth-to-water heat exchanger has also been tested, where outside air flows through pipes with circulating water buried in the ground. This system reduced heat stress as efficiently as fans and sprinkler systems but with a significantly lower electricity and water consumption. 
A floor cooling system typically uses underground pump-powered water pipelines under a thick layer of concrete, installed under the pig lying area. Access to a cool sleeping area has been shown to improve pig comfort, respiratory rate, feed intake as well as milk production for the females and the health of weaned piglets. 
Global warming affects the requirements for controlling the thermal conditions of livestock buildings. The application of adaptation measures may lead to increased costs (e.g., running costs, energy, and investments) of intensive livestock production systems, but should be balanced by their effects on productivity and animal welfare. 
 Godyń, D.; Herbut, P.; Angrecka, S.; Corrêa Vieira, F.M. Use of Different Cooling Methods in Pig Facilities to Alleviate the Effects of Heat Stress—A Review. Animals 2020, 10, 1459. https://iifiir.org/en/fridoc/142794
 Schauberger, G., Mikovits, C., Zollitsch, W. et al. Global warming impact on confined livestock in buildings: efficacy of adaptation measures to reduce heat stress for growing-fattening pigs. Climatic Change 156, 567–587 (2019). https://doi.org/10.1007/s10584-019-02525-3
 Evaporative Cooling, 27th IIR Informatory Note. https://iifiir.org/en/fridoc/138322