This isn't without evidence. The tomb chamber is sealed for years and contains many chemicals, over centuries, it's easy to form toxic miasma.
Many tomb raiders, after prying open the chamber of an ancient tomb, hurry inside to search for gold without guarding against the toxic miasma, which eventually leads to them suffocating and dying.
Moreover, some toxic gases are deliberately left in the ancient tomb by the tomb chamber's owner to ensure tomb robbers don't leave the tomb alive.
Some poisons are colorless and shapeless and, if infected accidentally, can cause a series of diseases.
Ancient Egyptian pyramids have legends of Pharaoh's curses, supposedly anyone who enters the pyramids is hunted by Pharaoh's ghost, ultimately killing them.
