I am doing a report on whether environmental regulation has an impact on goods exports for the UK (Using difference-in-differences). I have export data from 1990 to 2018 split by SITC codes. However, I was told that if I focus on the SITC codes which are more pollution intensive for the UK in terms of production, it can make my hypothesis stronger and can clearly establish a causal effect. But I am unsure as to how I can find which codes are the most pollution intensive for the UK. Is there any way to establish this properly?