I have heard that when you sit very long times every day that it is as bad for your health as smoking cigarettes. Especially, it is bad for your metabolism and especially the lungs get into a bad condition on an equal level. Is that true?
If we compare the average increase in all-cause mortality due to sitting (approximately 50% according to meta-study published on NHS) and the average all-cause mortality due to smoking (aproximately 300% increase according to CDC data), siting does not appear to be nearly as bad.
Yet, because of how broadly you worded the question it is possible that smoking is not as bad as sitting. According to the same publication by CDC that I linked above:
Quitting smoking before the age of 40 reduces the risk of dying from smoking-related disease by about 90%.
Therefore, for smokers who quit before the age of 40 on average the all-cause mortality due to smoking would be just 30%, lower than 50% for caused by sitting.
Regarding your questions of metabolism and harm to lungs. Bad posture while siting restricts maximum lung capacity therefore limiting oxygen intake. Additionally, siting decreases insulin action in blood, decreasing metabolism.
Research has linked sitting for long periods of time with a number of health concerns, including obesity and metabolic syndrome — a cluster of conditions that includes increased blood pressure, high blood sugar, excess body fat around the waist and abnormal cholesterol levels. Too much sitting also seems to increase the risk of death from cardiovascular disease and cancer.