## 4 The Central Role of Climate Control  
### Climate Control and Animal Welfare  
Animal welfare has always been associated with coping with the environment, as highlighted by the definition of animal welfare provided by [PERSON] [28]. In addition, the second domain of animal welfare proposed by [PERSON] [30] is focused on the environment, highlighting that uncomfortable or unpleasant physical features of the environment are negative for animal welfare. In this context, hence, climate control is fundamental since it provides adequate thermal and gaseous environments inside livestock houses. The impor  
Figure 2: Conceptualization of the relations between climate control and the considered domains of livestock production.  
tance of an adequate thermal environment in terms of animal welfare is also highlighted by the protocol for the overall assessment of animal welfare developed by the Welfare Quality Network [29]. One of the 12 principles at the basis of this protocol specifically regards animal thermal comfort.  
To understand the deep relation between climate control and animal welfare, the changes that happen to animals' metabolisms when the thermal environment is not adequate should be understood. Indoor effective environmental temperature is the main parameter that can be considered for evaluating if the thermal environment is adequate for animal farming. This parameter depends on air temperature, relative humidity and air velocity [77] and it has to be maintained within the thermoneutral zone (TNZ) to guarantee the animal's thermal comfort. When the effective environmental temperature is within this range, the animal is in thermal comfort and most of the intake energy is used for growth and production. When the effective environmental temperature is lower than the TNZ, the animal can suffer from cold stress--with potential hypothermia--and its rate of metabolic heat production must be increased by adopting different strategies, such as behavioral modifications or the increase of feed intake. By contrast, values of effective environmental temperature higher than TNZ can cause heat stress that can lead to potential hyperthermia. In this situation, the animal should increase its heat losses to maintain the thermal balance by adopting behavioral strategies, reducing feed intake or sweating [22].  
Another condition related to indoor climate that can jeopardize animal welfare is poor IAQ. The indoor air of livestock houses is characterized by high concentrations of different types of pollutants such as gases, vapors, bioaerosols and particulate matter. These pollutants originate from animals, feed, manure and from microorganisms that are associated with manure [37]. They can impair animal welfare with further negative consequences on health and productivity. To improve IAQ and to consequently enhance animal welfare, European regulation sets threshold limits for certain noxious gases in livestock houses. For example, NH\({}_{3}\) and CO\({}_{2}\) concentrations should be maintained below 20 ppm and 3000 ppm, respectively, in European broiler houses with stocking densities higher than 33 kg m\({}^{-2}\)[16]. Nevertheless, values over these limits were reported in commercial poultry houses. [PERSON] et al. [34] measured NH\({}_{3}\) concentrations up to 29 ppm in a broiler house in different periods of the year. NH\({}_{3}\) concentrations up to 39 ppm were found by [PERSON] et al. [38] in a broiler house in the Mediterranean area. The analyses performed in that work show that the previously mentioned thresholds of NH\({}_{3}\) and CO\({}_{2}\) were exceeded during 60% of the monitored period. The control of CO\({}_{2}\) concentration represented a mayor issue at the beginning of the production cycle, when broilers were chicks, while the control of NH\({}_{3}\) became a mayor issue as broilers grew.