FWIW the study most of the recent news is based on is
here
Its based on evidence from Denmark and lab studies and the relevant detail on page 6
Basically a stove installed before about 5 years ago would have 150 pm2.5 g pollutant per GJ house heating compared to 0.6g for gas boiler. Oil boiler has 0.1g. Open fire would have 1367g, wood pellet boiler has 55g and heat pump 1g.
So yes, the reduction over an open fire is considerable - 150g rather than 1367g. And I'm hearing people taking what they are hearing in the press to mean that burning in an open fire is somehow "safer" which is clearly not true. And I presume don't know if this is controlled for fuel quality - Mickey burning the half dried bits of last years ash tree is clearly going to emit far more PM2.5 than a controlled fuel with under 20% moisture content.