Final answer:
The most deadly product of combustion in fire deaths is toxic smoke, particularly due to the presence of carbon monoxide (CO) and hydrogen cyanide, both of which can inhibit the body's ability to utilize oxygen.
Step-by-step explanation:
The product of combustion that causes the most fire deaths is toxic smoke. Toxic smoke often contains carbon monoxide (CO), a colorless, odorless gas that can cause harmful health effects by reducing oxygen delivery to the body's organs and tissues. High levels of CO can lead to death. Toxic smoke may also contain hydrogen cyanide, which like carbon monoxide, interferes with cellular respiration and the body's ability to use oxygen. This interference can be lethal, especially in the context of smoke inhalation during a fire. While thermal energy and carbon dioxide are also products of combustion, they do not have the immediate toxic effects that carbon monoxide and hydrogen cyanide have.