## 4 The Central Role of Climate Control
### Climate Control and Air Emissions  
As just stated, gases, vapors, bioaerosols and particulate matter worsen IAQ of livestock houses, with detrimental effects on animal welfare. To avoid similar problems and to improve IAQ, contaminant concentrations are usually controlled through natural or mechanical ventilation, a strategy that has a dual effect. On the one hand, ventilation dilutes and removes contaminants, improving IAQ. On the other hand, ventilation can enhance the same air emissions, increasing the contaminants that are emitted inside and outside the livestock house. A clear example of this dual effect can be found, for example, in broiler houses. The high ventilation flow rates that are usually adopted in this type of livestock house during warm seasons decreases the NH\({}_{3}\) concentration, but, at the same time, these high ventilation flow rates favor the NH\({}_{3}\) volatilization from litter with a consequent increase of air emissions. By contrast, in cool periods, ventilation flow rates are considerably lower to minimize the ventilation heat losses. Consequently, NH\({}_{3}\) is not diluted and concentration increases [34]. This dual effect shows that techniques for emission reduction should be implemented in broiler houses. [PERSON] et al. [33], for example, proposed the installation of air circulation units for drying the litter and,hence, decreasing NH\({}_{3}\) and odor emission. [PERSON] et al. [36] proposed decreasing the emission of particulate matter through an optimized oil spraying method.  
A similar relation between the increase of ventilation and the consequent increase of air emissions can also be found in pig houses, especially in the ones that are equipped with a partly slatted floor. According to the analyses of [PERSON] et al. [32], ventilation air flow is one of the three main parameters that affect NH\({}_{3}\) and CH\({}_{4}\) emissions in pig houses, together with the type of rooting material and animal activity. High ventilation air flow rates entail high air speeds, a parameter that is positively related with the mass transfer coefficient from manure to air [78]. When air characterized by a high speed passes over the free surface of the manure, the emissions increase. [PERSON] et al. [39] found an inverse behavior between NH\({}_{3}\) concentration and relative humidity, and a direct relation between NH\({}_{3}\) concentration and indoor air temperature in weaning rooms, results that are similar to the ones of [PERSON] [35].  
Air emissions inside livestock houses could also increase due to the adoption of mechanical equipment for providing supplemental heating to the farmed animals. A commonly adopted strategy to provide supplemental heating in livestock houses is to place combustion air heaters directly inside the enclosure. This configuration is favorable since installation is cheaper and all the heat produced is released directly into the enclosure, with benefits from the energy and financial points of views. By contrast, this solution is detrimental for IAQ since the combustion fumes are exhausted directly into the enclosure, further increasing air emissions, mainly CO\({}_{2}\). Better solutions should be considered for providing supplemental heating without further increasing the air emissions inside the same enclosure. Valuable options could be the adoption of other technologies, such as electrical heating lamps or radiant floors, or exhausting fumes directly outside the enclosure.  
Finally, indoor climate conditions can affect air emissions in an indirect way since they have a strong impact on the urination and defecation behavior of reared animals. According to [PERSON] et al. [31], indoor air temperature is positively related to the pig urination and defecation frequencies that, in turn, affect NH\({}_{3}\) emissions.