A US study has linked eating cured meat like bacon and hot dogs with increased risk of lung disease.

The study is published in the American Journal of Respiratory and Critical Care Medicine and examines the link between frequent consumption of cured meats and impaired lung function in terms of the increased odds of chronic obstructive pulmonary disease (COPD).

Chronic obstructive pulmonary disease (COPD), of which emphysema and chronic bronchitis are the most common form (and often co-exist), is characterized by swelling of the airways.

According to the American Lung Association, COPD is the fourth leading cause of death in the US and more women now die from it than men. In 2003 it claimed 122,283 American lives.

Other studies, mostly on animals, have shown a link between nitrite consumption and reduced lung function.

Nitrites are added to cured meat such as bacon, hot dogs and cured ham as preservative, colour or as anti-bacterial agents. They are thought to generate reactive nitrogen species in the body -- molecules that cause structural damage to lung tissue, in a similar way to emphysema.

This is the first study to establish a direct connection between cured meat consumption and COPD.

The research team of four scientists was led by Dr Rui Jiang of the Division of General Medicine, Department of Medicine, at Columbia University in New York.

They examined data on 7,352 Americans aged 45 and over (average age 64.5 years) who took part in the Third National Health and Nutrition Examination Survey between 1988 and 1994.

The results showed that, even after adjusting for smoking, diet, age and other factors:

-- The more cured meat they ate, the poorer the participants' lung function.
-- Those who consumed cured meat 14 times a month were 78 per cent more likely to have COPD than those who ate none or very little.

Lung function was a measure of the amount of air a participant could blow out in one second (Forced Expiratory Volume for 1 sec, or FEV1). The total amount of air they could blow out after a deep breath was also measured (Forced Vital Capacity, FVC), but there was little effect on this measure.

Dr Jiang and colleagues concluded that:

"Frequent cured meat consumption was associated independently with an obstructive pattern of lung function and increased odds of COPD."

Critics of the study say that cured meat no longer contains the levels of nitrites that were present ten or twenty years ago, and therefore these results do not reflect today's situation. There are other sources of nitrites in the average diet, where only about 5 per cent comes from cured meat, they say.

Dr Jiang and colleagues did find that participants who ate the most cured meat were also more likely to smoke, be male, and of lower socio-economic status. They were also more likely to consume more calories, and eat less fresh fruit and vegetables.

However, the researchers pointed out that they adjusted for these factors. They said more studies should be done to find out if eating cured meat directly causes COPD or whether something else links the two.

According to the US National Library of Medicine, the leading cause of COPD is smoking, and 15 to 20 per cent of long term smokers develop the condition.

Long term tobacco smoking causes inflammation in the lungs and destroys the air sacs.

Apart from smoking, risk factors also include passive smoke inhalation, being male, and spending long periods of time in polluted atmospheres.

There is also a rare emphysema that afflicts a very small number of smokers and non-smokers alike, caused by a deficiency in the enzyme alpha-1 anti-trypsin.

Symptoms of COPD include: persistent shortness of breath (dyspnea) lasting from months to years, wheezing, finding it hard to exercise for very long, and coughing (with or without phlegm).