FOLLOWING earlier correspondence in your columns on the topic of smoking at Kai Tak Airport, I am delighted to see that the arrival hall has now been designated a smoke-free area, with ''No-Smoking'' signs on the walls, and a courteous sign explaining the new policy. This seems to have been introduced with little fanfare, but the Civil Aviation Department merits congratulations for this move. Not only does it create a pleasant, healthy environment for people (including babies and children) waiting to greet someone off a flight, but it also creates a good first impression of Hongkong for arriving passengers. Dr JUDITH MACKAY Director Asian Consultancy on Tobacco Control