A particle detector has a sample area of cross section no in excess of about 2 mm for containing environmental fluid, a light source on one side of the sample area for directing a collimated or nearly collimated beam of light through the sample air or water so that part of the light beam will be scattered by any particles present in the air or water while the remainder remains unscattered, and a beam diverting device on the opposite side of the sample area for diverting or blocking at least the unscattered portion of the beam of light and directing at least part of the scattered light onto a detector. The detector produces output pulses in which each pulse has a height proportional to particle size and a pulse height discriminator obtains the size distribution of airborne particles detected in the air or water sample at a given time from the detector output. The detector may also include a device for discriminating between biological agents and inorganic particles.