Engineering360: "Why Air Pollution from U.S. Manufacturing Declined During 1990-2008"

Very interesting article. I am a little unclear as to the meaning of "stringency of environmental regulation". Does this phrase mean that more environmental laws and standards were enacted, or that existing laws and standards were enforced, or a combination of the two? I would also like to know if the researchers measured the difference in plant CAPEX investments during this timeframe as an index of the degree to which companies either sought compliance voluntarily or did so in anticipation of being fined by the state and local EPA, or just wanted to adapt cleaner production methods in an effort to "get lean" and/or upgrade aging plants.

Finally the conclusion the author reaches in the last paragraph, that "stringency of environmental regulation for manufacturing firms was shown to nearly double between 1990 and 2008. This increase, rather than improvements in manufacturing productivity or trade exposure, accounted for most of the decreases in pollution emissions."...seems to contradict the author's opening statement that....

"The emission reduction is largely attributed to federal environmental regulations and attendant adoption of cleaner production methods"...which suggests that the two factors had an equal impact on the decline.