Which of the following air pollutants is the least hazardous to humans when they are present in small amounts? (A) Ammonia, NH3. (B) Carbon dioxide, CO2. (C) Sulfur dioxide, SO2. (D) Nitrogen dioxide, NO2. Or (E) hydrogen sulfide, H2S.
Air pollutants, including solid particles and gases, can interact with water in the eyes, nose, mouth, skin, and lungs. Sometimes this interaction causes no harm. But sometimes it produces new products which are harmful. The question asks which substance in the list causes the least harm to humans in small amounts? Let’s look briefly at each of the substances in the list and how they interact with water. Remember, a large percentage of the human body is made from water, so these gases can interact with water in many parts of the body.
Before we look at the options, we should recognize that carbon dioxide, option (B), is probably the answer since our bodies actually produce fair amounts of carbon dioxide naturally during the process of respiration. Thus, there is always some carbon dioxide in the cells and blood of our bodies. Our bodies do produce some of the other substances naturally, but in very small quantities as they are quite toxic.
When ammonia interacts with water in the body, ammonium hydroxide is formed, a weak base. This can dissociate into ammonium ions and hydroxide ions. The presence of hydroxide ions increases the pH of the bodily fluids slightly, making it more basic. When carbon dioxide interacts with water, carbonic acid is produced, a weak acid. And this dissociates partially into hydrogen ions and carbonate ions. Hydrogen ions interact with water further to produce H3O+ ions, known as hydronium ions. Excess hydronium ions decrease the pH of a bodily fluid slightly, causing it to become slightly acidic.
SO2 and NO2 react in a similar manner. SO2, when it reacts with water, produces sulfurous acid, a weak acid, H2SO3. And this dissociates partially to form hydrogen ions and sulfite ions. When NO2 interacts with water, a strong acid is produced, nitric acid, HNO3. Strong acids dissociate completely. In the case of nitric acid, H+ ions and NO3− ions are produced. NO3− ions are called nitrate ions. So, both SO2 and NO2 produce H+ ions and therefore hydronium ions. So, because NO2, SO2, and CO2 all produce hydronium ions, they all decrease the pH of bodily fluids slightly. In other words, they make bodily fluids slightly acidic.
H2S also produces H+ ions in solution and sulfide ions. So, H2S also makes bodily fluids slightly acidic. So, we have seen that these five substances can slightly alter the pH of bodily fluids. Also, ammonia, sulfur dioxide, nitrogen dioxide, and hydrogen sulfide all irritate the mucus membranes of the nasal passages, including the throat and lungs.
Because carbon dioxide does not really irritate these mucous membranes, we say that carbon dioxide is the least hazardous to humans when it is present in small amounts.