Experiments were performed to quantify the effects of droplet concentration and air velocity on the suppression and extinguishment of a boundary layer flame with ultrafine water mist (UFM). Unlike the traditional nozzles, UFM has very small droplet size (Sauter mean diameter, 3 mm) and forms laminar flow with low momentum. The results show that the UFM mass fraction needed to extinguish the flame decreases linearly by a factor of 7 as the air velocity is doubled. About 12% UFM is needed to extinguish the flame when the data are extrapolated to zero air velocity. Below the extinction concentration, the flame temperature is reduced slightly, but the local burning rate is suppressed significantly. In contrast, the spray-nozzle-mist (SMD range 15 to 50 mm, high momentum) enhanced the local burning rate at water concentrations below the extinction limit. Furthermore, the extinction concentrations are significantly lower with the spray-nozzle-mist than with UFM. The precise reasons for this are not clear but it may be due to better penetration of high-momentum droplets into the flame than the UFM. Unlike the UFM, the nozzle spray induces turbulence in the flame. Despite these differences, the UFM and nozzle mist data show that the extinction concentration exhibits a shallow minimum in the range of 25 to 40 mu m as the droplet size is increased.