The masks are nearly essential in Detroit right now if you need to spend any amount of time outdoors in the dim smelly murk. We've had our worst air quality in our history the past 2 days.
During our recent RS Best of Scandinavia tour people were succumbing to a cold like dominoes. It didn't appear to me that the masking was very effective as eventually even our guide caught it and had to miss several activities and barely made it to the farewell dinner. It was a particularly bad cold with severe nasal symptoms and terrible hacking cough. There's a number of tour members missing from group photos when they opted out of daily activity or stayed on the bus when it was too cold or rainy outside during photo taking.
Some people only brought the surgical masks, I'd recommend bringing the better quality masks. You might need them.