Works great! I don't always notice when I'm having an attack because it's less suddenly-can't-breathe and more a slow strangling of my lungs. I was on a trip recently and had a very bad reaction to the pollen there. It took me three days of struggling before I realized what was wrong. I know, what a dummy! But an oximeter would have helped so I got this one. It's perfect, exactly what I wanted, and it helps me monitor my heartrate for other conditions I have. :)