respiration rate - A hazardous gas usually enters a person's body through inhalation into their lungs. If they are breathing quickly, they will breathe in more of the gas than if they are breathing slowly. So their dose is higher if they are breathing heavily.
Knowing the source of a hazard will help the scientists to reduce exposure in the future, but will not help to determine how much a person has in their body.
hazard concentration - A higher concentration of a hazardous gas means a higher dose because there is more of the hazard to breath in.
frequency of exposure - A person exposed only once is likely to have a smaller dose than a person exposed many times.
length of exposure - A person exposed for a short time will have a lower dose than a person exposed for a long length of time.
properties of the toxin - Some gases are not easily absorbed by the human body and exposure does not lead to as high a dose as exposure to a gas that is easily absorbed.