If someone asks you if you mind them smoking by you, by all means you should say ‘Yes, I do mind."

The smoke they exhale from their cigarette endangers your health just as they are endangering their health

-even more. Those gases and poisons they are exhaling are pure and they are polluting the air you breathe...

上句主要是时态不理解，其他都使用了一般现在时，但是为什么 just as they are endangering their health

和they are exhaling are pure and they are polluting the air you breathe...都突然使用了现在进行时呢？

很是困惑。