The other major smoking-related killer is chronic obstructive pulmonary disease (COPD), a progressive and degenerative condition which eventually results in an inability to breathe; emphysema is a subset, as it were, of COPD.
In emphysema, the alveolar sacs which surround the alveoli merge into each other; they lose their elasticity and breathing in becomes more and more difficult. People don't often realize how affected they are -- 80 percent of COPD goes undiagnosed.
Sufferers simply assume that they are a little breathless, which comes with age, or perhaps lack of exercise -- and don't do anything about it. In fact, they are probably on a steep decline. By the time smokers reach 60, they can have a similar lung function to someone who has had asthma since childhood. Giving up smoking may halt that downward trend, but it can't reverse it.
"You never recover the lung function you have lost," West says.
After about 35, lung capacity and lung health decline anyway, so if they are already damaged, the decline is simply faster.
"People shouldn't be fooled into thinking that they can smoke for years and then, by stopping, remove all risk of smoking-related disease," Sandford says. "There will always be some residual risk."
It is true that not all smoking-related damage is irreversible. The risk of coronary heart disease drops by 50 percent in the first year after quitting smoking. Skin health also improves surprisingly fast.
"It is extraordinary how resilient the body is," West says. "It can put up with this insult day after day, year after year, and it forgives you -- until about the age of 35. After that, the clock is ticking, quite quickly. It's much more urgent than people realize."
All warn, however, that none of this counts as a reason not to stop smoking, and to do it as soon as possible.
"No matter how long you've smoked," Sandford says, "it's always worth quitting."