mtrainer90 posted on
another thread about HEPA filtration in the CRJs. The bottom line in that thread was that CRJ7 and CRJ9 had HEPA filters and CRJ2 did not, but the reason the lack of HEPA filter in CRJ2 didn't matter is that all of the air pumped into the cabin is fresh air from outside rather than recirculated.
Delta.com indicates that their planes either circulate fresh air or air through a HEPA filter
Air onboard is recirculated every two to six minutes with fresh, outside air or through HEPA filters that extract more than 99.99% of particles, including viruses.
I realize you're talking about the E175 and not the CRJ, but based on all the above, I'd say it's probably safe to assume there is a HEPA filter, or otherwise the air is fresh and not recirculated. Perhaps
mtrainer90 can give you a definitive answer?