Along with the protocols to determine housing system-specific emission factors, reduced building-specific NH3 sampling strategies for fattening pigs were proposed by Vranken, Claes, Hendriks, Darius, and Berckmans (2004) and later refined by Dekock et al. (2009). In the final protocol, a linear model containing ventilation rate, mean weight of the animals and inside and outside temperature, measured at specific times, was used to model the NH3 emission from a building. In total, four measurement periods (2 before day 70 and 2 after day 70 in the fattening period) per fattening period were needed to get a good estimate of the NH3 emissions (i.e. a maximum deviation of less than 10% between the measured and simulated NH3 emission). To get an EF for the building, three fattening periods had to be monitored (Dekock et al., 2009 and Vranken et al., 2004).