The actress departed the role of Vera Duckworth after 33 years in 2008 because she found herself becoming "stressed" on set and feared she was "letting people down".
"My breathing was so bad I thought I was dying," What's on TV quotes her as saying. "I'd smoked for many years and I'd always had a bit of a cough and a splutter but I was getting worse and worse and I was struggling to breathe.
"I was frightened of letting people down when I was filming. I would cry sometimes because I was so stressed.
"I couldn't walk more than a few steps without having to sit down and I was so tired I couldn't even learn my lines."
Dawn, who has since become an ambassador of the British Lung Foundation, also called for more people to get "checked over", noting that she would have been forced to quit Coronation Street earlier if she hadn't.
"As long as you get the right medication you can have a life," she said. "If I hadn't got treatment then I'd have had to leave Corrie years before."
Dawn made a one-off appearance as Vera in a 2010 episode that saw her on-screen husband Jack (Bill Tarmey) pass away from cancer.