Human Nature: Science, Technology, and Life.

How Dangerous Is Outdoor Smoke?

If you want to argue for parkwide smoking bans based on asthma or on an analogy to noise pollution, go ahead and make that case. But let's not cloud that debate by invoking the general harm of secondhand smoke. Studies of secondhand smoke have indeed moved outdoors. Their findings support restrictions on lighting up within a few feet of other people. But they don't warrant more than that.

More here.