My experience is that wearing a mask doesn't stop the reactions from happening. I also do not get relief from using benedryl if it reaches a full blown athsma attack. I have found for me that the liquid benedryl (children's) gets in my system much faster than the pills. Unfortunately a bottle does not last long when used for adult doses.
Plus it is not a sin to get up and walk out or miss mass due to health reasons.
I guess I am less concerned about it being a sin than my actually missing the message from the service. So often when I am visiting a different Catholic Church I find the Holy Spirit has led me to that particular church. I plan to check before the service starts to see if there is a cry room in case I need it.