Three retrofit technologies for tailpipe and crankcase emissions were studied determine their impact on the particulate matter levels in bus cabins. The catalytic options evaluated were the Johnson & Matthey's Continuous Regenerating Technology (CRT), the Environmental Solutions Worldwide's Particulate Reactor for tailpipe emissions; and the Spiracle Crankcase Filter system for blow-by emissions by Donaldson. Using the CRT and the Particulate Reactor showed significant reduction in particulate matter. The use of the crankcase resulted also in a reduction in particulate matter with the adverse effect of an increase on CO levels at the exhaust. This is an abstract of a paper presented at the AIChE Annual Meeting (Salt Lake, UT 11/4-9/2007).