From what youve told us, your parents are likely to be allergic. This is because their immune system overreacts when theyre exposed to something that can trigger sneezing and swelling of the nasal membranes. One way to help them avoid any further complications would be by ensuring that the room stays clean and dust free so as not to cause build up in their lungs. People who suffer from asthma should avoid exposure too since it will increase the chances of an attack