Cigarette smoke is a toxic mixture containing around 4000 different chemicals including a range of carcinogens, irritants, and toxins.¹ It is therefore no surprise that inhaling cigarette smoke, either actively as a cigarette smoker or passively through exposure to exhaled and sidestream smoke from other smokers, is bad for health.¹ In fact, active smoking kills more people in economically developed countries than any other preventable cause, and currently accounts for over 100 000 deaths (or about 20% of all deaths) each year in the UK.² Worldwide the annual death total is currently close to 5 million.³ Passive smoking is also a major problem, causing at least 12 000 deaths each year in the UK,⁴ or about 2% of the current annual total, and probably a similar proportion in other developed countries. On these figures alone it should be clear that we have sufficient evidence to conclude that preventing exposure to cigarette smoke, either active or passive, should be a fundamental priority of all clinical and public health practice. We already know what the main health effects of smoking are, or in relation to passive smoking, are likely to be. So do we need more research defining …