Oximeter. They're cheap, you can grab one on Amazon for $40. My friend who mailed me the medicine supplies (who is married to a doctor) insisted I have it and that I measure it regularly.

> Is this something that people with rescue inhalers also typically have?

I'm not sure. I usually get a rescue inhaler prescribed roughly once a year to help get over a cold. I have light asthma and bad allergies. The breathing here in Denver is definitely more challenging in general as well.