The BBC just published an article on it here: https://www.bbc.com/news/world-asia-india-50258947 

Half way down is a video they've done called: "Do pollution masks work?". Without being too explicit of those they tested or it would look like an advert, they just mention this one has the most protective features which is available in five colours: https://www.amazon.co.uk/gp/product/B000OYFUDY/ 

Ugly beasty because it really does stop both macro and micro pollution with a full facial seal and one-way valves, but if it's wear that or hack and cough my way through my holiday - I'll put up with looking like Bane, a villain from Batman - where it's needed. It's meant for cyclists in cities where you are chasing buses and their diesel exhausts but it has a place in cities with extremely severe pollution as well.

I've just ordered one for my upcoming trip to India with Just You, which starts in New Delhi.

The Beeb do go as far as to say not to bother with cheap surgical masks as they are a complete waste of time and don't stop any pollution from being breathed in. They're there to trap microbes in surgery, not stop pollution particles getting through when out and about - mainly because they are not sealed to the face.