H tells us how much heat energy will be given off or will be required for a particular chemical reaction. It doesn't tell us whether the reaction will occur or how fast. Endothermic reactions are usually not a great safety hazard. However, because the reaction draws heat from its surroundings, the reaction container may become cold and cause condensation or ice to form. This can be a safety hazard if the materials involved react with water.
Many common chemicals undergo exothermic reactions.
The page on oxidation has an example of an even more exothermic reaction. As temperature increases, the rate of chemical reactions generally increases as well. This can be especially dangerous if the material reaches its flash point or autoignition temperature , at which point a fire or explosion could occur. Therefore, it is very important to know when a chemical reaction can generate excess heat and to take appropriate measures to deal with this. Examples include slow mixing, using a cooling bath or avoiding that reaction.