Workers' paradises have always been the worst polluters because the polluters, the smelters, the coal/nuclear power plants, the manufacturers, all are owned by the people...the government. Unlike relatively free economies, the government conspires with polluters, rather than regulates them.
It was so in the USSR; it was so in Eastern Europe; so it is in China. If Cuba did anything else beside tobacco, cane, and bananas, it would be a major polluter, I am sure.