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Like any chemical process, combustion releases substances. We will see in further detail what these substances are.
Gases are the most dangerous combustion residues, for human health. Combustion residues depend on the original chemical composition of fuels and, as most fuels are carbon-based, two highly harmful gases for the human body are often produced: carbon dioxide and carbon monoxide. Water vapour is not contemplated as a combustion gas, since, as soon as the original temperature is restored, steam goes back to its liquid state.
But these are not the only kind of gases that can be produced during a combustion.
This gas is produced during fires caused by petroleum derivatives, oils or fats. It is highly toxic and irritating. A concentration higher than 10 p.p.m. can be deadly
It is produced during fires caused by materials containing nitrogen, like wool, silk, acrylic, phenolic materials or melamine resins. It irritates eyes, nose, throat and lungs. A half-an-hour long exposure at a concentration of 0.25-0.65% is enough for the human body to suffer serious permanent damage. In some cases, death ensues.
Carbon dioxide
Carbon dioxide is an asphyxiant gas which is produced in great amounts during generic fires that develop with a high concentration of oxygen. A consequence on the human body is the hyperventilation. Consequently, if the fire produces other toxic gases, the presence of carbon dioxide contributes to a better inhalation of these. A concentration of 10% carbon dioxide is lethal.
Carbon monoxide
Carbon monoxide is produced during fires caused by carbon compounds and a small quantity of oxygen (therefore, especially in enclosed or badly ventilated spaces). Carbon monoxide directly alters the structure of blood, bonding with haemoglobin and preventing it from correctly delivering oxygen to tissues. Therefore, in this case, death is by asphyxia. A concentration of 1.3% carbon monoxide is enough to cause loss of consciousness in a few moments' time and death in a few minutes' time. Breathing air with a concentration of 0.05% carbon monoxide can lead to death in 3 hours' time. It is absolutely odourless, colourless and flavourless.
Dinitrogen tetroxid
Dinitrogen tetroxid, or nitrogen tetroxid, is a highly toxic gas, a concentration of 0.02-0.07% dinitrogen tetroxid in the air leads to death in a few minutes' time. It is a reddish-brown nitrous vapour, and it is produced during fires caused by materials containing nitrogen, like nitrocellulose, ammonium nitrate or other organic nitrates.
Hydrochloric acid
Hydrochloric acid is produced during fires caused by plastic materials, which often contain chlorine. It is corrosive to metals, indeed, in more than one occasion corrosion of metals has been observed in the area of the fire. It is extremely toxic, 0.01% hydrochloric acid in the air needs only half-a-hour's time to kill a human being. It can be easily perceived, as it immediately irritates mucosa and it has a pungent smell.
Hydrogen cyanide
It is mostly generated during combustion of wool, silk, acrylic, urethane or polyamide resin, when the oxygen sustaining the process is scarce. It is a highly toxic gas, a concentration of 0.3% hydrogen cyanide is enough to lead to death, and it has a distinctive smell, like bitter almonds. Cyanide ion directly reacts with cells and inhibits their respiratory function. Due to its toxicity, wearing PPE during intervention in areas where this gas has been released is essential.
Hydrogen sulfide
If the fuel that caused the fire contains sulfur, one of the gases produced by combustion is hydrogen sulfide. Indeed, the peculiar smell of this gas, similar to rotten eggs, is caused by sulfur, smell that disappears in a few moments' time. A concentration between 0.04% and 0.07% hydrogen sulfide in the air is more than enough to cause vertigo and vomiting. A higher concentration of hydrogen sulfide attacks the nervous system and inhibits it, besides giving shortness of breath. Sulfur can be found in materials like wool, rubber, leather, meat and hair.
Like hydrochloric acid, phosgene is generated in fires of plastic materials, or other materials containing chlorine, especially if the fire developed in an enclosed space. It smells like musty hay and it is colourless. 0.003% phosgene in the air can be lethal if breathed for a half-a-hour's time. Nonetheless, its effects do not appear immediately, but 24-72 hour after the exposition. After the inhalation, it reacts with the water that can be found in the respiratory tract and it breaks up into carbon monoxide and hydrochloric acid; death ensues by a mix of internal bleeding and respiratory failure.
Sulfur dioxide
Gas created during a fire caused by a fuel containing sulfur (wool, rubber, leather, meat, hair, etc.) and a great amount of air. Normally it is produced in small quantities, unless the fire is directly caused by pure sulfur. This gas irritates the eye and respiratory mucous membranes, and a concentration of 0.05% is enough to be dangerous.