From Katherine Huntly of the British Lung Foundation, in a letter in today's Guardian:
Smoking just one cigarette, even with the car window open, creates a greater concentration of second hand smoke than a whole evening smoking in a pub or a bar.
And from ASH's latest smoking-in-cars 'fact sheet':
Opening a window does not reduce the levels of secondhand smoke in a car to a safe level as the smoke simply blows back into the vehicle, often lingering for hours.
As Frank Zappa once said, it's not getting any smarter out there.