“…Manure and feed are regarded as the main sources of PM in the pig house (Cambra-López et al, 2011) so twice a day manure removal shortens the retention time of manure in the pig house, thereby substantially reducing the PM produced from manure. In addition, fogging, spraying or sprinkling clean water may reduce the concentration of dust in the air (Takai and Pedersen, 2000;Aarnink et al, 2011;Costa et al, 2014). Therefore, the lower PM concentrations inside pig houses operated with the "cleaning the manure dry" manure removal system are expected and reasonable.…”