Quant Technologies LLC has developed a growing line of Water Condensation Particle Counters (WCPC) to measure the number concentration of submicrometer sized particles in air.  These particle counters can detect particles from 2.5 to < 3000 ng quickly which make them ideal for a myriad of applications including indoor air quality measurements, environmental and atmospheric research, inhalation and exposure studies, and exhaust/exposure studies.  These particle counters are marketed exlusively through TSI Incorporated: www.tsi.com