What are the causes of air pollution and smog in China? Why not 20 years ago?

In the final analysis, it is the result of environmental destruction and economic development ignoring the environment.

China has a large population. With the development of economy, there will be more and more factories and cars, and people's awareness of environmental protection is not strong.

There is also no specification or standard (even if there is, it can be said that it is not up to standard, even if it is up to standard or strict, there are problems in actual implementation).

The previous friend is right, there is no evaluation and testing technology. But I think it should be related to people.

There are relevant technologies and norms, but people's awareness is not strong and their enforcement is not enough, so it is.