[Dan(9698)]

...as it is heavier than air, like chlorine and carbon dioxide.

These both must be lighter than air. They get up where the ozone layer exists and cause Global Warming and deplete the ozone which causes skin cancer.

[alpo]

The molecular weight of "air" (80% nitrogen -N2- MW 28 and 20% oxygen -02-MW 32) is about 29. The molecular weight of chlorine gas is 35 and carbon dioxide -CO2- is 44. Therefore Chlorine and CO2 are a bit heavier than air.
Gasses mix together so the heavier chlorine will be found in time in the upper part of whatever vessel it is in and if that vessel is the atmosphere it will be detected in the upper reaches.

When 1 ton chlorine cylinders are stored at water treatment plants, there are sensors and ventilation ducts at floor level. Fans on the ceiling will blow the gas out of the floor vents if there is a leak. Chlorine was particularly effective in WWI trenches as it would "fill" shellholes and trenches and drive the surviving troops out into sniper and machine gun fire. Occasionally workers die in wineries when they enter an open top tank that has fermented wine. The tank can be full of invisible carbon dioxide and you don't know it until you pass out and fall on the tank floor. Then someone will jump in to pull the victim out and end up dying too.