Most of the particle emissions from vehicle exhaust can be removed by installing a closed particulate filter in the exhaust system. It is important to distinguish between the effective closed particulate filters and the ineffective open particulate filters. Closed particulate filters remove more than 99% of the particle emissions for all particle sizes (PM10, PM2.5, and
PM0.1). Open particulate filters reduce only the largest particle fractions by 15-30% and the reduction of ultrafine
particles is poorly documented (see page 15).