Biofilters have been considered as an effective technology for the abatement of odour and more recently, bioaerosols at enclosed waste management facilities. However, to aid the improvement of exposure assessments through longer duration sampling and better modelling protocols, and enhancement of future health impact studies, it is imperative to develop a better understanding of biofilter design and effective performance monitoring techniques especially if they are to continue to control all emissions and achieve their full potential. This study aimed at investigating the impact of gas residence time on the performance of biofilters in terms of Aspergillus fumigatus and total mesophilic bacteria reductions using four pilot-scale biofilters operated in a materials recovery facility based in Leeds, UK. A six stage Andersen sampler was used to measure the concentrations of both groups of bioaerosols in the airstream before and after passing through the biofilters and these were compared with the background concentrations measured upwind of the site. Results suggested that gas residence time did not appear to have any statistically significant impact on bioaerosol removal efficiency. Nonetheless, the biofilters achieved removal efficiency up to 90% for total mesophilic bacteria and 95% for A. fumigatus, provided that the inlet concentration was high (103 – 104 cfu m-3), which is the case for most waste facilities. The performance was highly variable at low inlet concentration (worst case: resulting in reductions of -144% and -167% for total mesophilic bacteria and A. fumigatus, respectively), suggesting that biofilters had the potential to be net emitters of bioaerosols. In conclusion, this study shows that biofilters designed and operated for odour degradation can also achieve significant bioaerosol control in waste gas.