The Netherlands’ most polluting companies have not reduced their emissions by making improvements and becoming more durable but by reducing their production levels, news website Nu.nl reported on Friday.

Figures from the Dutch emissions authority NEa show that while overall carbon dioxide emissions are down 13% over two years, the very biggest polluters are producing more of the greenhouse gas for every tonne of product than a year ago.

In total, 356 companies are classed as major polluters and fall under the European emissions trading system ETS. Together, they are responsible for around half the carbon dioxide production in the Netherlands. 

The NEa calculates a carbon coefficient, based on carbon dioxide emissions and actual industrial production. These coefficients show that while a small majority of the firms have become less polluting without reducing production levels, some of the biggest polluters are actually emitting more carbon dioxide per tonne of product.

Heavy industry is facing a major challenge to become more sustainable, NEa director Mark Bressers said. “These new figures show that a lot has to be done to ensure carbon dioxide emission can fall further while maintaining or increasing productivity.”

The oil, gas and chemicals industries are among those which have become more inefficient, Nu.nl reported. For example, Shell’s giant chemicals plant in Moerdijk has cut carbon dioxide emissions by 13% over the past two years. But its coefficient has worsened from 19% to 31% below the EU baseline, Nu.nl reported.

Ministers are working on agreements with the Netherlands 20 most polluting firms, but the process has taken two years so far and only one deal, with salt company Nobian, is nearing completion.