Even low-levels of soot, the black powder that comes from burning things and collects in chimneys, can be deadly to older people, reveals a new study. A four-year pollution study, which was a first of its kind, carried out by researchers at the Health Effects Institute, found that at least 143,257 deaths could have been prevented between 2006 and 2016 if the standard and allowed limit of particulate matter (PM 2.5) had been tightened to 10 micrograms per cubic metre.