**3. Odours**

The odours are a significant source of gaseous pollution in broiler housing, derived from anaerobic and aerobic microbial activity during litter decomposition. The litter is considered the primary source of odour in broiler housing. The malodorous compounds are originated by organic particulate matter, volatile fatty acids, sulphurous (H2S, mercaptans) and NH3 (**Figure 2**) [37]. The main odorous compounds emitted on broiler farms are dimethyl sulphide, dimethyl disulphide, dimethyl trisulfide, nhexane, acetic acid, 2,3-butanedione, methanol, ethanol, 1-butanol, 2- butanol, 1-octen-3-ol, 3-methyl-1-butanol, 3-methyl-1-butanal, acetone, 2- butanone and 3-hydroxy-2-butanone [38].

Odour emissions from litter are complex due to the existence of multiple odorant sources within litter (i.e., fresh excreta, friable litter and cake), formation and emission of numerous odorants, and significant spatial and temporal variability of moisture content, porosity, pH, ventilation airflow, temperature, humidity and bird activity (**Figure 2**) [9, 38].

There has been limited studies of management strategies that reduce the formation and emission of odorants from broiler litter, mostly focussed on the perspective of reducing NH3 emissions by air scrubbing, misting, filtering, ionising, oxidising and dispersing technologies [28, 38, 39]. In addition, strategies are focused on the perspective of reducing odours in the air through fogging technologies combined with the use of masking agents, counteractants, neutralizers and surface-enhanced absorption agents (**Table 3**) [28, 38, 39]. Dunlop et al. [40] selected management strategies with expected effectiveness to reduce odour emissions from broiler litter, being maintaining dry and friable litter; *in situ* aeration of litter; in shed windrowing/pasteurising of litter reuse; litter acidifying additives; litter adsorbent addition (activated carbon, silica gel or zeolite); and enzyme addition combined with heated incubation.
