Many factors affect the distribution of a toxic substance but water or fat solubility is very important. The distribution of a toxic substance determines its concentration at a particular tissue and therefore the number and type of cells exposed to high concentrations of it. Metabolism or biotransformation does not necessarily result in less toxic compunds. Carbon monoxide is present wherever there is incomplete combustion of carbon compounds. Hydrogen sulphide might initially be detected by its smell at low concentrations but it paralyses the sense of smell and can effectively become odourless.
Absorption can also occur through the skin.
At low concentrations these gases poison cytochromes and cause the rapid onset of headache, dizziness, vomiting and confusion. At high concentrations they are very rapidly lethal. Synaptic transmission from a nerve cell to another cell such as a muscle cell in many situations relies on acetyl choline. Organophosphate and carbamate pesticides inhibit acetylcholinesterase and lead to accumulation of acetylcholine at sites of neuromuscular transmission causing weakness of muscles, and paralysis including of respiration.