There are several apps that will give you updated pollution levels. I was there two weeks ago and kept being told about the terrible "fog" in Agra and how there can be no pollution in Agra because the factories all closed long ago... A simple google search will tell you otherwise. The apps reported very high pollution levels with a 2.5 count above 300. Nobody will fuss at you if you wear a mask.

February will be better, but in December the high speed train took four hours to reach Agra due to fog (yes, there is also fog - it's just that it isn't all fog).