(3) Use an efficient respirable dust recirculation filter. All the cab field demonstrations used recirculation filters that were 95% efficient, or greater, on respirable-sized dusts. Laboratory experiments showed an order of magnitude increase in cab protection factors when using an 85%- to 94.9%-efficient filter on respirable-sized dusts as compared to using no recirculation filter [NIOSH 2008]. Laboratory testing also showed that the time for interior cab concentration to decrease and reach stability after the cab door is closed was cut by more than half when using the recirculation filter.