Diesel particulate emissions are less than 2.5 micron. This size range particulate are major contributors to visibility-related problems in both urban and rural areas, as in the Denver Brown Cloud. Diesels are currently estimated to make up 10 to 20 percent of Denver's haze. Particulates in the diesel size range are also inhalable, capable of traveling deep into the lungs and being retained for long periods of time. As diesel particulates contain large quantities of organic materials, long term health effects from exposure to diesel particulates are of concern. It is estimated a diesel I/M Program will reduce diesel particulates from affected vehicles by 10 percent.Proposed changes would impact all diesel motorists and owners who have diesel motor vehicles registered or principally operated along Colorado's Front Range. Existing regulations affect approximately 10,000 diesel trucks in fleet operations, whereas, the new I/M program will affect all other diesel motor vehicles, (passenger cars, light trucks, and non-fleet trucks) estimated at 30,000 vehicles.Under the regulations, all diesel motor vehicles registered, operated, or maintained in the AIR Program area must undergo annual emissions (opacity) inspections, prior to vehicle registration or re-registration. Pursuant to the enacting legislation, “fleets” are allowed to self-inspect their vehicles (under Division guidance and auditing) whereas, all other diesel vehicles must be inspected at a state licensed testing facility.It is estimated that 30 percent of the fleet, 9,000 vehicles, will fail the initial inspection, and require repairs for compliance. Based on previous Department of Health testing, repair costs will average $450.00 per failing vehicle.