system
Given the following post, respond as if you're commenting on Reddit:
human
If I'm in polluted air or there's an awful smell, is it better to breathe through my nose or my mouth, to have as few molecules reach my lungs as possible?
gpt
It honestly depends on what is in the air. Is is particles or chemicals? For larger particles (think dust) , breathing slowly through your nose would be best. Your nose is built as a rough filter. 

For a chemical, think paint thinner or another solvent, it wouldn't matter. You need specialty air filters to remove these (think gas mask). 

Smells, which are generally chemical, will enter your lungs through either your nose or mouth. Your body has no real method to filter out things like this. 

If you are concerned about particals, wear a N95 or N100 rated mask. For solvent or smells, which would not be considered particles, you will need a respirator rated for solvent. 

Hopefully this helps.