About 21,000 Australians are expected to be hospitalised this winter by an often undiagnosed respiratory condition.

And health experts say it is set to cost the health system nearly $78 million.

Chronic Obstructive Pulmonary Disease (COPD) affects one-in-five Australians over 40 but, according to The Australian Lung Foundation, as many as half are unaware they have the condition.

Foundation Professor Christine Jenkins said this was due to COPD - which attacks the lungs and causes breathlessness - commonly being confused with other conditions like asthma or the flu.

Many people also attribute symptoms of COPD to poor diets, sedentary lifestyles and the long term effects of smoking, she said.

"Unfortunately people do have their own explanations which means they often don't go to the doctor to find out more," she told AAP.

Prof Jenkins is urging people not to ignore the symptoms and seek proper medical treatment.

"It's a destructive inflammatory lung disease... It's really important if you are getting breathless to ask your doctor if you can have a breathing test."

In economic projections, released on Tuesday, the foundation said it expects 21,000 people to be hospitalised with the disease during June, July and August this year.

COPD is also predicted to cost the health system nearly $78 million - more per case than cardiovascular disease, osteoporosis or arthritis, the foundation said.

Source: http://www.currentaffairs.ninemsn.com.au/health/8481977/lung-disease-could-hospitalise-21000