Biochar is becoming a commercial biomass-derived product that is transported, stored, and applied to land for environmental management. However, no information is available about its flammability that significantly affects how biochar can be handled. Given that biochar can have very different properties depending on how and from what it is produced, flammability may also vary significantly. The flammability of biochar and its dependency on biochar properties were quantified for a range of biochars produced at different pyrolysis temperatures and as a function of time after production. None of the studied biochars (34 samples stored for at least two years under argon gas) qualified as flammable substances, assessed using the applicable UN method. The majority of biochars (67%) had no combustion front propagation distance at all. Almost all of the studied fast pyrolysis biochars (71%) had higher combustion distances, whereas most slow pyrolysis samples (80%) did not combust. The combustion of stored biochars increased with the amount of volatiles (r(2) = 0.27, p < 0.05, n = 11; dominated by fast pyrolysis biochars: r(2) = 0.62, p < 0.05, n = 5), typical of biochars produced at lower temperatures. In contrast, the combustion of biochars within minutes of production was higher for biochars made at 723 K (450 degrees C) than 623 K (350 degrees C), but decreased to negligible levels within 1 h. Short-term flammability may be a function of the amount of free radicals and surface areas that can react with oxygen, whereas long-term flammability after storage may be a result of the potentially flammable volatile matter and some still weakly explained mechanisms for high-ash dairy feedstock. (C) 2014 Elsevier Ltd. All rights reserved.