When? I don't know Houston, but I recall in MN a refinery that made the whole area stink for 10 miles around. 15 years latter I went by and the air was great even when driving buy the main gate. Soon after my brother in law got a job at that refinery and he told me that for a years they decided the EPA fines for releases were a cost of going business, but when management decided to clean up they were quickly able to root cause and fix all the issues that caused "releases." Houston can clean up as well, but since I've never been to that city I can't say if things have changed.