It's also been great to see Beijing Capital International Airport join the leading modern airports in the world, like London Heathrow and Los Angeles International, by making indoor smoking rooms a thing of the past.

I think even those who doubted whether the law could or would be effectively enforced would have to admit to being pleasantly surprised.

Of course, there will continue to be naysayers - those who say that it will never work, or never last. This happens all over the world. But with a tough law, accompanied by continued strong enforcement, there is no reason to think that Beijing will be any different to other great cities in the world like New York, London, and Sydney - where "smoke-free" is now the unquestioned norm.

I'm confident of this, because we know that there is strong public support in Beijing for public places being smoke-free. And the international experience suggests that public support will increase over time, even among smokers themselves, as the public comes to realize the benefits of the law.

And with good reason: the benefits of smoke-free laws are substantial. Exposure to second-hand smoke is deadly. There is no safe level of exposure. Most people don't realize that second-hand smoke causes dangerously high levels of indoor air pollution. The PM2.5 reading when just three people are smoking in a restaurant is likely to be around 600; five smokers, and the reading is likely to top 1,200 - far worse than the outdoor air pollution on even a heavily polluted day.

The war against outdoor air pollution is a complex, long-term task. Fixing indoor air pollution caused by second-hand smoke can happen overnight by making public places 100 percent smoke-free. Beijing has now taken its place alongside other great cities around the world in doing this - and the residents of Beijing are now breathing easier for that.

The author is World Health Organization representative in China.