Other filters and chemical "scrubbers" catch oxides of sulphur and nitrogen that would otherwise cause acid rain.
The report also found that smokestack scrubbers and other equipment, which accounted for upwards of one-third of total plant expenses to operate, were idled to save costs.
In the United States and Europe, black carbon emissions have already been reduced significantly by filters and scrubbers that are now required to fight local particulate air pollution.
