Here’s a guide to how it works. The Air Quality Index reflects the density of five pollutants in the air: ground-level ozone, particulates, carbon monoxide, nitrogen dioxide, and sulfur dioxide.

and so for equal volumes of metal and air then the metal would be heavier. Density is used to make a fair comparison between materials. Density is a measure of how tightly the mass of an object is ...

Groves, G. V., Proc. Roy. Soc., A, 252, 16 (1959).