Droplet size and velocity measurements from commercial "fogger" type pepper spray products

Commercial pepper spray devices are available that deliver either a coherent liquid stream or an aerosol from the canister. Information is unavailable in the literature regarding either the spray characteristics (i.e., the droplet diameter, velocity, and number density) or the canister-to-canister variability of these devices. Consequently, their performance in delivering the active agent to the target is not well characterized and the amount of material delivered as small, potentially harmful, particles is unknown. This investigation used phase Doppler interferometry (PDI) to measure the size and velocity distributions of aerosol-type pepper sprays. This information was used to obtain preliminary information on the amount of smaller diameter droplets present in pepper spray products currently on the market. This preliminary information could then be combined with toxicity information (expected to become available) to determine whether the potential hazard is sufficient to warrant further study. Thus, characterization of the sprays (i.e., both droplet size and velocity) at the target location is important for this purpose. Each canister was fired repeatedly to discharge its contents in 1 s shots at 1 min intervals until the PDI could not detect droplets. Four different sets of commercially available pepper spray canisters were studied to document the variation in the aerosol diameter distribution from shot to shot. The results indicated that there were significant differences in the spray characteristics for the different canister sets. The number of shots per canister varied among the sets of canisters. The droplet mean diameter was fairly constant per shot for three of the four canister sets (averaging 54.0 µm with a standard deviation of 2.4 µm for all shots from these three sets) until nearly all of the canister liquid contents were expelled. At this point, the values of the droplet mean diameter, streamwise velocity, and number of droplets decreased significantly. The droplet diameters detected varied from a few micrometers (at the detection limit of the optical arrangement) to about 120 µm. For some groups, the diameter distributions were bimodal with peaks at about 10 µm and 40 µm.