How can you possibly know that?
The lung cancer death rate in Thailand is 32 per 100,000. The death rate for lung cancer in Malaysia is less than 20 per 100,000.
Everyone sees all types of vehicles in Thailand belching smoke. That is why there is so much air pollution. Chiang Mai and Chiang Rai hit an AQI of 500 during the smoke season, when the WHO standard for good air quality is 50.
In contrast, one never sees smoke from vehicles in Malaysia. Currently, AQI in Kuala Lumpur is 45. Population 8.6 million. In Chiang Mai, it is 75. Population 1.2 million. Draw your own conclusions.