Abstract: A pathogen detector has a sample area for containing environmental air, a light source on one side of the sample area for directing a collimated beam of light through the sample air so that part of the light beam will be scattered by any particles present in the air while the remainder remains unscattered, and a beam blocking device on the opposite side of the sample area for 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 sample at a given time from the detector output. An alarm signal is activated if the number of particles within a predetermined pathogen size range of around 1 to 7 ?m exceeds a predetermined normal level.