Then you may want to think twice about Hong Kong as well -- thanks to the vast industrial zones on the mainland around it, pollution there can get worse than Bangkok, although summers are usually better than winters.

I don't find the pollution in Bangkok to be bad at all, and neither does an asthmatic friend of mine, but YMMV. Singapore is certainly cleaner, and a visit in the summer would be before forest fire haze season (and there hasn't really been any in the past two years).