The aim of this study was to review research papers with gas emission data from livestock buildings that were published between 1997 and 2015. The review focused on three gases: NH₃, N₂O and CH₄ and two animal species: pigs and dairy cows. The results of the review are presented in different units, which makes it difficult to compare the data. For this purpose, the gas emission factors were converted to 1 LU (livestock unit = 500 kg).
The median of NH₃ emission factors for pigs (45.6 g·day^-1·LU^-1) was almost twice that of dairy cows (26.7 g·day^-1·LU^-1). For N₂O the emission factor median for pigs (3.2 g·day^-1·LU^-1) was more than twice that of dairy cows (1.5 g·day^-1·LU^-1). Also for CH₄ , the median of emission factors for dairy cows (302.5 g·day^-1·LU^-1) was more than three times higher than the value for pigs (85 g·day^-1·LU^-1). The variation in the gas emission factor values for pigs and dairy cows is large. This may be due to the following reasons: geographical location, animal species, feed composition, housing and ventilation system or time of measurements. Therefore, there is a need to continue gas emission monitoring research in order to more precisely determine the values of these gas emission factors for pig and dairy production facilities. Measurement procedures should be standardized including the number of measuring days/months, frequency of sampling, measurement equipment and unit of gas emission factor units. Using common methodology for measuring gas emission will allow better comparisons between emission factors for livestock buildings and housing systems and between countries and animal species.