The second level of difficulty only explored about 1/2 of the level at once. I chose the best ones that could reduce pollution, but it was still not enough.
Dream chasing boy
Your field of view is too small, you can fight few monsters, and it is inconvenient to move around. If your equipment has 2 fields of view, the infection will not exceed 300 on a rainy day.
distant journey
Killing monsters will also clean up the pollution. If you can control the pollution to 80%, it will be barely enough.
sunshine in rain
Pollution will deduct your stamina at the maximum, but when you enter the battle and build up armor or shield for yourself, then when you move, your shield and armor will be depleted first, which can alleviate it a bit.