\" Ultrafine pollution particles come from many sources — cars, fires, machines — but are so small and transient that measuring exposure to them is difficult. But other types of burning — wood fires, burning leaves, secondhand smoke from cigarettes, for example — also produce ultrafine particles. Ultrafine particles are so tiny that they can penetrate the blood-brain barrier and be absorbed into brain tissue. AdvertisementAn Unregulated DangerDespite all that, federal air pollution standards don't yet set limits for ultrafine particles. Given uncertainties in the health evidence for ultrafine particles, and limitations in the available monitoring data, a separate standard for ultrafine particles was not established in the last review of the PM NAAQS, completed in 2012.