Yes, heavier than air gasses can be very dangerous. Many of them don't provoke a buildup response like CO2 does in your lungs so you'll asphyxiate without knowing it. Large refrigerant cooling systems in enclosed spaces can be a system. I think there have been some guys working on some large systems that died from a gas leak that floods a room. https://www.hvacschool.org/2017/08/the-deadly-inhalation-risks-that-hvac-mechanics-and-installers-face/ Also seen videos of people throwing dry ice in a pool for the fog effect...Which pools on the surface and people can't get their head through to air.