Pollutant Emissions in Livestock Buildings: Influence of Indoor Environment, Rearing Systems, and Manure Management


 The issue of air pollutants from livestock buildings is prevalent in the literature. Because they and their emissions impact both animal production and livestock building users as well as the outdoor environment. This paper aims to compile and review data available in the scientific literature on the types of pollutants for a better understanding of their generation form, their distribution according to the kind of animal, and the main factors affecting their generation and concentration, i.e., the rearing system, the indoor microclimate, and the manure management. The elevated generation of pollutants in animal buildings is tied to the dense occupancy in this industrial activity. The indoor air quality is defined according to the type of livestock in animal housing, considering its welfare needs, and the types and concentrations of pollutants generated as a function of the family of animal and the management used in production. The main gases generated are CH4, CO2, H2S, NH3, N2O, in addition to particulate matter and airborne microorganisms such as fungi and bacteria that very negatively affect the health of animals and users of the animal buildings. Furthermore, knowledge about the main contaminants generated, the form of generation, their origin, their concentrations, and their distribution throughout the shed is essential to achieve a permanent and adequate indoor air quality and, with that, a high-quality product that will lead to high production yield without neglecting animal welfare.


Introduction
FAO data (FAO 2020) show that from 1961 to 2018, the world population increased by 147% while the total meat production (all types) increased by 380%. Another fact is that livestock production represents 50% of the total agricultural product and supports many developing countries (Herrero et al. 2016;FAO 2017).
The animal production gures are a direct result of human consumption. As of 2019, chickens, pigs, goats and sheep, and cattle and buffaloes were reared for meat production amounting to 27.5 x 10 9 (billion) live animals (of which chicken were about 23 billion), while almost 234 million cows were used for milk production. In contrast, in the egg production sector, there were 7.5 billion laying hen (FAO 2020). A signi cant intensi cation in livestock farming production has occurred because of the increase in both livestock buildings and indoor animal crowding in search for higher productivity. Other factors that enable the production growth can also be cited, such as using feed of higher nutritional value, improvement of pharmaceuticals, routine vaccination, and improvement of the infrastructure and feed e ciencies (Leip et al. 2015).
Collectively known as Animal Feeding Operations (AFO) and occurring within facilities where animals are concentrated or con ned, these factors contributed to increasing animal production (Ramankutty et al. 2018). Currently, such facilities represent the most extensive, worldwide method for industrial-scale livestock production (Mallin et al. 2015).
The impact of livestock production on surroundings is also relevant, causing effects on air, water, soil, biodiversity, and climate change, resulting in increased local and global environmental concerns (Leip et al. 2015). Therefore, reducing the pollutant emissions from management in animal buildings, emphasizing Indoor Air Quality (IAQ) improvement for the AFO, can now be considered the main research topic (Ni 2015).
Massive efforts have been invested in the basic research to achieve new conceptions on air pollution.
Several research projects have focused on identifying, quantifying, characterizing, and modeling air pollutant emissions in animal buildings through improving sampling and monitoring devices and developing mitigation methods. Results have yielded practical knowledge about what determines IAQ in different types of livestock production (Ni 2015).
Some introductory examples of research that lead to this knowledge can be considered. ) monitored the environment of three different laying-hen housing systems: conventional cage, enriched colony, and typical aviary, and concluded that the IAQ was similar in conventional cage and enriched colony, both with ammonia and particulate matter concentrations bellow the typical aviary. On the other hand, (Chai et al. 2018) found that while cage-free housing better agrees with natural behaviors of hens (foraging, dustbathing, wing-apping, etc.), IAQ was lower than in the conventional system. (Ni et al. 2012a) tested two types of laying-hen houses (high-rise and manure-belt) and veri ed the in uence of the house design resulting in worse IAQ in a high-rise as compared to manure-belt, where a strong correlation was observed between IAQ and climate parameters (temperature and air ow rate) and animal conditions, in uencing the results.
Different building materials also exposed to potential in uence in IAQ. (Wang et al. 2011) found that selecting oor material became critical for IAQ. They compared two commonly-used systems in pig houses: fully slatted oor and deep fermented litter and concluded that the IAQ was worse in the case of the slatted oor.
This article aims to compile and discuss information regarding the in uence of the indoor microclimate, the rearing system, and the management of animal manure on the Indoor Air Quality in livestock buildings, focusing on atmospheric pollutant emissions from intensive animal production (Table 1). environmental impacts that reach the outside by ventilation  Enteric fermentation and manure management are the main sources of GHG emissions from animal production. In 2018, 46% of CO 2 emissions, 78% of CH 4 emissions and 6% of N 2 O emissions in agriculture from enteric fermentation + manure management (FAO 2020). In Spain, livestock contributed over 35% of all CH 4 emissions in 2017, of which 75% came from cattle (62% from meat cattle alone) (Gobierno de España 2019).
Many papers reported emission rates. The main pollutants described in animal buildings are airborne microorganisms, CH 4 , CO 2 , H 2 S, NH 3  . Poultry production emissions are higher than any other animal production, mainly because of a dense animal occupation, and constitute the major environmental problem for poultry farming (Nicholson et al. 2004) . In contrast, while poultry buildings are a signi cant source of CO 2 , CH 4 , NH 3 , and N 2 O emissions generating from the bedding, animal excreta, or uric acid decomposing into urea, followed by NH 3 and CO 2 volatilization promoted by urease enzyme (Rotz 2004), CH 4 and N 2 O emissions from poultry facilities usually are lower than that of cattle or pig production (Groot Koerkamp et al. 1998); swine production buildings usually show high concentrations of NH 3 , CO 2 , and PM, that have been found to affect negatively the health of both animals and humans (Wathes et al. 1998 Thus, the type of contaminants (and, indeed, also their effects in both animals' and worker's health) will differ according to the type of both animal production and management in the animal buildings, although their global effect must also take into account how much each type of production and management represents in the sector. For example, pork meat is more consumed than poultry across the world (FAO 2011), and therefore their contribution to the global emission form NH 3 becomes even more signi cant.
While a reduction of meat consumption would most certainly lead to a corresponding reduction in emissions, current expectations are that consumption will instead grow alongside world population and expansion of indoor animal production, with a projected increase in global meat consumption by 70% by 2050, mainly concentrating in developing countries with more intensive animal production (FAO 2011).
To counter this trend it is necessary to develop strategies to reduce pollutants in the livestock production building, where air quality is worse due to higher emissions per square meter (Nicholson et al. 2004) .
We will discuss below the main air pollutants found in livestock buildings, considering the effects on animal health and wellbeing, production e ciency, and the subsequent environmental impacts.

Airborne microorganisms: fungal spores and bacteria
Airborne microorganisms (mainly bacteria, fungi, actinomycetes, viruses, pollen, and some archaea) are omnipresent, lifted from the soil, water/seawater, vegetation, and other places (Stetzenbach et al. 2004;Zhai et al. 2018) One of the most common airborne microorganisms is fungal spores, which can be hundreds of times more frequent than other particles like pollen grains (Ebner et al. 1992;Takahashi 1997 High rates of airborne microorganisms occur in animal building and their impact is not restricted to the buildings themselves, as they can spread through natural air ow, affecting the IAQ and increasing the regional health risk (Vittal and Glory 1985;Lacey and Crook 1988;Hanhela et al. 1995;Seedorf et al. 1998;Huijskens et al. 2016) . Indoor animal housings in uence the transmission of airborne microorganisms signi cantly and may contribute to contamination of industries of food processing (Geornaras et al. 1996 Fusarium species are frequently found in animal feed. (Hanhela et al. 1995) detected airborne spores of Fusarium species during grain handling from 32 farms in Finland. While concentrations were low, they found Fusaria in 77% of grain and feed samples. A large variety of trichothecenes (a group of mycotoxins) from Fusaria have been identi ed from different types of cereals for animal feed in different geographical regions (WHO 1990).
Fecal contamination is another well-known bacterial problem that some authors noticed. In chickenslaughtering facilities, the presence of Escherichia coli in chicken carcasses, Staphylococcus aureus in slaughtering environments, Pseudomonas aeruginosa in food processing environments are pathogens related to hygiene habits of employees (Donnelly 1994;Geornaras et al. 1996;Eisel et al. 1997;Shale 2004;Lues et al. 2007). (Lues et al. 2007) alerted to a high rate of airborne microorganisms measured, highlighting the importance of maintaining a low microbial level before the processing stage.
Both fungal spores and bacteria usually attach to solid particulates, although they can also be found as individual bacterial particles (Zhao et al. , 2016 . In animal buildings, airborne microorganisms usually occur in feed zones, animal bedding, and manure (bedding + excreta), where manure is the most important.
A signi cant concern about these aerial contaminants is, in many cases, their relevant effect on the health of animals and farmers, as they can cause diseases such as allergic reactions or asthma. Hence, poor IAQ and building emissions of airborne microorganisms are key indicators of workspace health for farmers, animal welfare, farm e ciency and productivity, food safety, and environmental impact (Amon et al. 2007;Zhao et al. 2016;Costantino et al. 2020), and a reduction of bioparticle levels to ensure healthy and safe conditions at the animal production workplace is as desirable as, and likely leading to, safer exhaust air from animal buildings. Table 2 condenses the main aspects of airborne microorganisms. Table 2 Airborne microorganisms in livestock buildings.

Origin
• Moldy hay and foods.
• Animal feed operations.
Characteristics • Size = 0.65-3.3µm • Individual particles or clusters or attached to particulate matters.

Facilitators
• Wet and humid conditions induce decomposition of raw organic materials.
• Unhygienic feeding trough and bedding.
• Animal movement and activities.

Effects
• Nasal and ocular diseases through mucosa contact.
• Hay fever and other allergies by particles >10µm contacting the nasopharynx.
• Asthma and other allergic reactions by particles <10µm reaching the lower airways and lungs.
• Weak immunity, slow growth, and low feed conversion e ciency.

Methane (CH 4 )
In 2018, agriculture produced more than 142 million tons of methane through burning, cultivation activities, manure management (7%) and most importantly, enteric fermentation by bacteria in the digestive tracts of animals (71%) (FAO 2020). This scenario affords a deep concern because CH 4 is a GHG emission, and the livestock production does not stop growing.
Enteric fermentation is a natural process inherent to the largely anaerobic nature of digestion (especially in ruminants), and emissions will depend on the population size and trophic habit of livestock. On the other hand, manure management prompts a temporal succession of microbial processes, where substrates are converted into volatile fatty acids, CO 2, and hydrogen (H 2 ), increasing the temperature of the manure, and converting these products into methane (Hellmann et al. 1997

Origin
• Anaerobic degradation of organic matter.
Characteristics • CH 4 emissions vary with feed quality and intake and among animals of the same age and in the same herd.
• CH 4 production favored by lack of oxygen, high temperature, a high level of degradable organic matter, high moisture content, a low redox potential, a neutral pH, and a C/N ratio of between 15 and 30.
• Swift removal of manure reduces CH 4 emissions.

Facilitators
• Increased animal activities, mainly feeding that leads to digestive action.
• Higher temperatures on manure stored.
• Nutritional factors: feed concentrate composition, maturity of harvested forages and type of silage.
• No direct negative effect on livestock.

Carbon dioxide (CO 2 )
CO 2 in animal production is a considerable problem when, in livestock con nement buildings, the production is overly dense, i.e., too many animals sharing, and breathing in, a con ned space, however large. CO 2 can also be originated from manure breakdown, although for both cases, breathing and According to (Gerritzen et al. 2007) , when the instantaneous CO 2 concentration reaches 2,4%, effects can be noticed in broilers. Still higher concentrations may lead to more severe health problems such as gasp and convulsions. However, lower concentrations held during longer exposure times could also affect the poultry health; for example, when broilers are exposed up to 6,000 ppm of CO 2 for two weeks their bodyweight is depressed and late mortality increases (Olanrewaju et al. 2008) .
To minimize these problems, regulations were established, and concentration limits were assigned. On the other hand, in swine production with a high-density of animals, in addition to exhalation by pigs CO 2 comes from manure breakdown (Philippe and Nicks 2015) . In manure, according to (Jeppsson 2000) and (Wolter et al. 2004), CO 2 may have originated from three sources: 1) the rapid hydrolysis of urea into NH 3 and CO 2 catalyzed by the enzyme urease; 2) the anaerobic fermentation of organic matter into intermediate volatile fatty acids, CH 4 and CO 2 ; and, 3) the aerobic degradation of organic matter . Table 4 summarizes the characteristics of CO 2 in animal production. Table 4 Carbon dioxide in livestock buildings.
• Anaerobic fermentation of organic matter.
• Aerobic degradation of organic matter.
• Easily handled and stored.
• Essentially non-toxic at normal levels.

Facilitators
• Seasonal and diurnal activity patterns.

Effects
• Decreasing of the oxygen concentration.
• Cause gasp and convulsions in broilers.
• Loss on weight and increase in mortality in broilers.

Hydrogen sul de (H 2 S)
As a key component of the sulfur cycle, H 2 S is a colorless, potentially harmful gas (although in very low concentration has low effect) produced in nature through the anaerobic breakdown of sulfate by bacteria. Nevertheless, hydrogen sul de can be produced from human activities through various industrial practices and by the degradation of sulfur-containing protein in mammals (EPA 2003).
In livestock production, H 2 S usually derives from manure breakdown (anaerobic decomposition) through two distinct ways: 1) mineralization of organic sulfur compounds; and 2) reduction of oxidized inorganic sulfur compounds (EPA 2003).
Generally, low H 2 S concentrations are easily perceived, and long or extend gas exposure are taken as toxic and acutely dangerous to humans and animals: injury with chronic exposure at 10ppm and serious injury or death at > 500 ppm ( In livestock production, pig rearing is known as the animal production which has severe problems with H 2 S. And, in ruminants, the generation of large quantities of hydrogen sul de depresses ruminal motility and cause severe distress to the nervous and respiratory systems (Kandylis 1984) .
Details of H 2 S in animal production are shown in Table 5.

Origin
• Anaerobic reduction of sulfate by bacteria (manure).
• Degradation of sulfur-containing protein in mammals.
• Toxic: one of the most dangerous gases.
• Lower air ow rate.

Effects
• Injury and death in critically high concentration.
• Lesions of respiratory and digestive system.
• Severe distress of nervous system.

Ammonia (NH 3 )
The microbial decomposition of the organic part of the manure is the main source of ammonia in animal houses. NH 3 is generated from animal excreta (urine and feces) present on the oors of the buildings, When mixed into the atmosphere, the ammonia lifetime tends to be short ( ve days or less), and it is generally located near its generation site (Blunden et al. 2008 Ammonia is the main contaminant in poultry buildings. Its high capacity to latch on to other particles and substances because of its sharply hydrophilic base can make it pervasive, decreasing health, welfare, and Indoor swine production also suffers from the consequences of poor IAQ by ammonia contamination, through respiratory diseases in piglets and farmworkers, and seriously impacts ecosystems as well Details on the origin, characteristics, facilitators, and effects of ammonia in indoor animal production are presented in Table 6.

Origin
• Microbial decomposition of organic compounds.
• Manure and bedding material.
• Deposits of urine and feces.
Characteristics • Attaching to ne particulate matter.
• Urea is converted to ammonia by the enzyme urease.

Facilitators
• Characteristics of the manure.
• Air ow characteristics above the manure surface.
• Higher temperatures (same air ow rate) and lower air movement.

Effects
• Reduction of weight gains.
• In broilers: ocular damage, mucosal in ammation, enhances susceptibility to respiratory diseases and bacterial contamination of the lungs.
• Higher expression of gene inhibitor growth and breast muscle development.

Nitrous oxide (N 2 O)
Although N 2 O origins are still in need of much research, it is suggested that, worldwide, more than 65% of  On the other hand, even though it may represent a small percentage of all emissions as compared to other gasses, N 2 O is a strong GHG, having a global warming potential almost three hundred times higher than that of CO 2 and a long residence time (EPA 2010) and its emission from animal manure occurs in all animal buildings globally (Table 7). Therefore, nding ways to mitigate its production through manure management becomes an important task. Table 7 Nitrous oxide in livestock buildings.

Origin
• From ammoni cation of urea in manure, the ammonium produced is transformed by nitrifying bacteria under the conditions of su cient supply of oxygen (nitri cation).
• Nitrates in nitri ed slurry experiment denitri cation to gaseous N 2 O.
Characteristics • Signi cant greenhouse gas emission and consequently global warming and climate change.

Facilitators
• N 2 O is originated from manure decomposition process by bacteria.
• It depends directly on composition of manure, storage time and type of manure management.

Effects
• No direct negative effect on livestock, but strong driver for global warming.

Particulate matter (PM)
PM is composed of ne airborne solid and/or liquid particles containing oxygen, carbon, silicon, phosphorus, nitrogen, and other substances (EEA 2020). Normally PM is classi ed according to their size and the most common categories are 10, 2.5, or 1µm aerodynamic diameter which are usually known as PM 10 , PM 2.5 , and PM 1 respectively (European Commission Publication 1999) Table 8.
The electrostatic attraction on PM causes particle agglomeration and may signi cantly alter both size category and attached content, such as hazardous matter like bacteria and/or viruses added on PM (Harry 1978) . Therefore, PM might become a hazard. The health effects of PM have been exhaustively studied, and no completely safe level of PM has been found (WHO 2013) .

Origin
• All movements of solid materials (feed, bedding…).
• Coming from outside through the opens.
Characteristics • Fine solid or liquid particles.

Facilitators
• Ventilation and air movement.
• Animal movement and activities, mainly feeding.
• Density of indoor animals.
• Age of the animals.
• Temperature and relative humidity.
• Poor performance in racing horses.

Volatile Organic Compounds (VOC)
A One of the biggest problems associated with the presence of VOC in a rural industry is their generally unpleasant odor, which causes discomfort of workers and neighbors (

Origin
• Vaporization of molecules containing carbon.
• Manure storage is a major source of odor causing VOCs.
Characteristics • VOC-odor is composed from miscellaneous chemicals.
• Contribute to tropospheric ozone production.

Facilitators
• Presence of PM in the air.
• Presence of organic chemical products.

Effects
• Malodorous and ozone production. demand more e cient air ow through the shed to remove heated air and allow fresher air in. Thus, the emissions will likely distribute more evenly throughout the building but will also be exhaled from livestock buildings in larger quantities, leading to important environmental impacts in surrounding areas.
Rural sheds are usually equipped with air cooling systems to avoid losses in productivity from poor animal welfare when temperature rises. These systems are installed when natural ventilation alone is not able to reduce thermal impacts on production. On the other hand, when the T of the region is low and the buildings require maintenance of the internal heat, i.e., a warmed environment, the ow of fresh and cold air along the indoor building tends to be drastically reduced. In indoor environments, higher T and RH are associated with the higher generation, release, and dispersal of fungal spore as observed by (Herrero and Zaldivar 1997) in cattle sheds. (Blunden et al. 2008) con rmed the seasonality in aerial pollutant emissions in pig houses, particularly NH 3 and H 2 S, emphasizing the in uence of the air ow through the building, which tends to be lower during the cold seasons than during the warm seasons. Ensuring outdoor air exchange in livestock con ned production is always essential to reduce temperature and relative air humidity, and to renew internal air evacuating contaminated air. This air ow maintains IAQ and therefore, promotes animal welfare and productivity.
Adequate openings to natural ventilation are thus desirable in animal buildings, but when these are not possible or enough, mechanical ventilation should be considered to allow su cient air ow through the shed according to IAQ requirements in animal production.
In cold climates, animals may not require as much fresh air to reduce temperature and achieve welfare, but nonetheless a renewal air ow is necessary to remove pollutants. On the other hand, in warm climates and hot and humid climates, a speci c ventilation program must be designed to avoid poor IAQ associated to high T and RH. This higher ventilation rate can also assist in diluting pollutant concentration along the sheds, such as e.g. bioparticles as observed by (Zhao et  In open buildings, air exchange rates depend on both indoor parameters, such as temperature gradient and air ow, and outdoor parameters, like wind speed and surrounding topography (Ngwabie et al. 2009) .
These authors observed wide spatial and temporal variations in the concentration of CO 2 , NH 3 and CH 4 , inside a naturally-ventilated barn . (Bjerg et al. 2013) modelled the concentration and spreading of NH 3 above manure in livestock houses as a function of ventilation rate, air inlet conditions and temperature.
A signi cant in uence of temperature and air ow (especially air ow momentum and intensity turbulence) on the rates of NH 3 release from manure has been observed (Arogo et al. 1999;Ye et al. 2008;Rong et al. 2009;Saha et al. 2011). However, inside the building, air ow turbulence and natural wind variation reduce the accuracy of velocity data, and induce uncertainty about the air ow gradients and turbulence above surfaces that can potentially release pollutants, such as manure, bedding, and slurry (Bjerg et al. 2013).
As explained earlier, in a cold climate or weather environment livestock buildings reduce air exchange in order to maintain thermal comfort but must ensure enough internal air ow (called minimum ventilation rate) to control IAQ.
When minimum ventilation is used as hygiene air in animal houses, the focus should be placed on the worst pollutant emission, as it should afford the best relative IAQ improvement, rather than on some ready measurement. For example, in horse stables the minimum ventilation is usually established only to keep RH or CO 2 levels within a threshold. However, this can result in dangerous levels of NH 3  However, the period of the day did not seem to in uence NH 3 concentration in an enriched cage laying hen facility, while differences in CO 2 emissions were low in a study by (Alberdi et al. 2016).
Feeding intervals within a given type of management or even a feeder area when animals are free to access feed signi cantly interfere with the production of particulates and move part of the feed to the oor or bedding in these areas, even becoming the main factor for higher PM emission rates in swine In dairy production, feeding times of the cows also condition the development of the pollutant concentration rate because of increased animal activity. A correlation between gas emissions and feed programs in dairy buildings exists (Ngwabie et al. 2011) .
Different production systems may lead to variable emission rates. In a study where different laying-hen houses were considered, (Zhao et al. 2016) observed that in aviary house systems, total bacteria concentrations and emission rates were much higher than both conventional cage house and enriched colony house systems.
In equine buildings, the inhalation exposure to PM was higher in stabled horses versus no-stabled horses, principally when the activities of the horses were walking and eating (Vandenput et al. 1997;Fleming et al. 2008;Nazarenko et al. 2018).
Trying to mitigate high concentrations of PM in horse stables, (Nazarenko et al. 2018) investigated an alternative polymeric material, woody PET, for stall bedding. However, the use of woody PET resulted in increased PM concentration over natural straw, which is still considered the best bedding material for stables even when horse activity is high.
Several nutritional factors affect the rate of enteric CH 4  N 2 O is not likely to be produced from manure stored indoors in pits beneath the slatted oors, though this is the most common excreta storage system in pig and cattle farming (Monteny et al. 2001). N 2 O emissions are expected in housing systems that are based on solid manure. In these systems, animal excreta are either already in the form of solid manure (poultry) or are being collected in, e.g., straw or wood shavings (pig and cattle). However, manure deposited on the oor and/or pit promotes the release of NH 3 , though ammonia production and concentration will depend on the characteristics of the manure, the rearing, the microclimate in the building, and the air ow above the stored manure surface (Bjerg et al. 2013).
In recent decades, bedded systems have been used in swine buildings, achieving better welfare, odor nuisance, and GHG emissions (Philippe and Nicks 2015) than in the most common system based on a slatted oor where the animal excreta fell on a pit used to store slurry, although other authors could not In free-stall barns, manure is normally removed every few hours to once a day by scraping or ushing.
With this rapid removal of the manure, CH 4 does not have time to build and indoor emissions are low. In contrast, with a slatted oor manure accumulates in a pit under the oor from a few weeks up to several months, and in a bedded pack barn, manure accumulates on the oor for a few months as more bedding material is being added over the winter to absorb moisture (Rotz 2018). Under the aerobic and anaerobic conditions found within the pit or manure pack, N 2 O and CH 4 emissions become much greater, respectively. Manure also accumulates on an open lot, but the manure is spread in a thinner layer where the more aerobic conditions induce less GHG emission (Rotz 2018). On the other hand, the effect of high compaction of the bedding may also become less favorable for the generation of contaminants, especially those aerobic processes that require oxygen.
Displacement of manure is often related to the release of GHG. If there is not enough ventilation in the shed, manure movements within the shed will thus elevate indoor contaminant levels. Manure buildup promotes emissions and requires additional manure handling, compounding the problem. In any case, GHG will eventually nd their way to the atmosphere through building exhalation. On the other hand, frequent removal of manure from the building may reduce indoor emissions and enhance IAQ.

Discussion
There is a signi cant, demand-driven increase in the production of animal by-products worldwide. This is only possible through a considerable improvement and intensi cation of livestock production in controlled and closed buildings. However, emissions of pollutants such as airborne microorganisms, CH 4 , CO 2 , H 2 S, NH 3  Low IAQ affect both livestock and workers, and can also cause serious environmental impact problems on the neighborhood when the gases are exhaled from animal buildings. Achieving desirable IAQ following the environment guidelines of each country requires identi cation of the generated pollutants according to the type of animal production and its functional characteristics such as breed and welfare needs, as well as local climate, management actions or rearing, typology of building/shed and environmental control actuators, and analysis of the data collected through continuous monitoring.
Mitigation procedures must consider all those factors and data for successful implementation.
According to our literature review, the most cited production variables that lead to changes in the characteristics of IAQ are the rearing system, indoor microclimate, and manure management. Although these three variables tend to be quite interrelated in a systemic view, a speci c view of each variable is essential when selecting mitigating actions.
The production of pollutant gases derives from complex chemical reactions that strongly depend on the availability and quantity of speci c compounds and on the conditions in the manure and its environment such as temperature, moisture content, presence of oxygen, surrounding air ow, physical characteristics of the deposits (porosity, compaction, speci c surface, adsorption potential, etc.). Hence, the quantity of emissions is not just a direct function of the volume of manure generated by the livestock, but also of where and how it is generated, including microclimate conditions (T and RH) of the manure storage sites, mixture with other components such as bedding or water, the dynamics of air ow through the building, and the effects of gas dispersion.
Solid particulate matter and microorganisms, attached or not to PM, also depend on all three factors (rearing, microclimate, manure). Fresh air intake for renewal and cleaning is also a source of PM in the indoor building. Microorganisms, on the other hand, depend on the organic matter and environmental conditions to reproduce and are linked to both the hygiene condition of the sheds and the health condition of the herds.
Herd management, bedding composition, and building typology and design (spaces for movement of the livestock, feeders and drinkers, manure storage areas for short and long periods, etc.) are also factors that in uence qualitatively and quantitatively the production of air pollutants.
Manure management, especially bedding turning, removal, storage, and manure-bedding mixing operations, causes important concern about emissions in indoor livestock production. Most mitigating solutions aim at the physical-chemical treatment of manure, especially when they are deposited outside the shed. However, certain herding practices and manure-bedding mixtures can result in lower environmental impact even inside the building.
The microclimate conditions of the shed are decisive for the development and propagation of indoor air emissions. Temperature and air ow are the predominant, interdependent microclimate variables, as ventilation conditions are primarily determined by the livestock's sensitivity to adverse thermal conditions, both high and low. These variables exert a signi cant in uence on the production of pollutant emissions from manure, depending on the rearing system in place. A ventilation program in an animal building that does not consider emissions and their buildup could lead to two undesirable results: 1) dispersion of contaminants from stored manure throughout the house, and 2) in cold climates, elevated indoor contaminant levels through the drastic reduction of ventilation to minimize heat loss. Both outcomes may noticeably affect the health of animals and workers.
A new trend to collaborate in the hard work of mitigating emissions in animal buildings may be emerging when designing livestock sheds with low environmental impact. Investments in renewed barns can be associated with innovation and sustainable thinking. The relationship between sustainability and wellbeing issues seems to be a path that highlights the care with the IAQ in animal buildings (Galama et al. 2020) that is certainly worth exploiting.

Conclusion
Animal by-products generated in high-e ciency, intensive farming systems in closed and controlled buildings, lead to the generation of signi cant amounts of waste per unit area of animal occupation. In addition to the solid waste that is produced in large volumes, the production of livestock in walled buildings results in a similarly important volume of aerial pollutant emissions, such as gases, solid particles, and microorganisms. The aerial pollutant emissions affect the con ned animals, the users responsible for handling the herd, and the natural environment outside the shed due to the signi cant exhalation of dirty air from inside the building that occurs constantly due to the necessary air renewal.
The type and amount of emissions for any livestock class will be strongly conditioned by 1) the indoor environment; 2) the rearing system applied, mainly the management of the herd and the typology of the shed; and 3) the manure management, especially when stored for a long time inside the building.
These three sets of variables could be analyzed separately to verify their speci c impact on the production and level of each type of pollutant. However, all three sets are usually interrelated, and therefore a joint analysis is desirable in order to account for synergies and cancellations. Mitigating solutions for low IAQ in animal buildings should thus be also based on a systemic relational study of the variables considered here. The authors declare that no funds, grants, or other support were received during the preparation of this manuscript.

Competing interests
The authors have no relevant nancial or non-nancial interests to disclose. Breakdown of the selected articles according to the type of document, the pollutants considered/researched, the animals studied, and the factor of in uence considered Scheme of pollutant production in animal building