More than 4,000 elementary, middle and high schools across Korea have shut their doors as the country’s student population shrinks, new data shows.
According to the Ministry of Education’s latest figures, revealed on Sunday by Rep. Jin Sun-mee of the ruling Democratic Party of Korea, since 1980, 4,008 schools under 17 regional education offices nationwide have closed as of March this year. During the period, the number of enrolled students decreased from 9.9 million to 5.07 million.



Consider that there might not be a single unifying explaination and instead a number of compounding factors.
Right, which is why I specified most important factor. I put it in the hypothesis to be more prominent, but perhaps I should have bolded it?
Fair enough, I meant to subtilty disagree with you on the hypothesis that pollution is the leading factor and instead suggest that it may not be the leading factor, as I have been taught that socioeconomic factors like education do most of the explaining. But that is not what I wrote of course.
But I’m no expert on the matter so you might very well be right either way. The topic appears to be well studied though, but I haven’t gotten around to reading any papers