Heating elements are used in some synthesis. Heat can also be generated by two or more chemicals or elements reacting with each other.
A violent chemical reaction between chemicals, water and/or air could result in an explosion.
Acids are used in some synthesis process of producing illicit drugs. Physical contact with a contaminated item or area could result in a chemical burn.
Lethal phosphine and phosgene gases, and other non-lethal but irritant or obnoxious odor gases.