The Toxicity Gauge fills during the following three types of inclement weather.
While heavy rain is falling, enemies and plants won't appear on your radar. Heavy rain also causes the player's footsteps to be more audible unless they are sneaking.
In smog, enemies and plants won't appear on your radar, and food will rot more quickly. Smog tints the screen purple, and it's possible for smog to appear at the same time as heavy rain or heat.
Food rots more quickly in the heat. The screen turns orangeish until bright sunlight disappears.