Ultrafine particle (those less than 100nm, or 0.1µm in size) are produced through combustion activities and are persistent in all ambient air. Concentrations of these tiny particle can be very high near emissions sources (for example, near busy roads), and both acute and chronic exposure are believed to lead to health problems. As part of the new US EPA near-road ambient monitoring network, ultrafine particles (UFPs) will be monitored to determine concentration levels and further study health effects related to exposure.