This article from JAMA (https://jamanetwork.com/journals/jama/fullarticle/2846867) tells the story of figuring out why the rate of heart disease had decreased precipitously. Although lead had long been known to poison the heart, it hadn’t been considered a factor in heart disease. Investigation attributed most of the unexplained decline to the decrease of lead, not just from gasoline but also paint and safer recycling of batteries.
‘In the US, coronary heart disease mortality rose in the 1930s and 1940s alongside expanding production of leaded gasoline. By 1968 - the year production peaked - coronary heart disease accounted for 1 in 3 deaths.
‘Coronary heart disease mortality then began a steep and sustained decline. During this same period, production of leaded gasoline and blood lead levels dropped in parallel. Hypertension prevalence also declined - from roughly 1 in 3 adults in the mid-1970s to 1 in 5 by the early 1990s.
‘In 1978, more than 80 physicians and scientists gathered in Bethesda, Maryland, at what became known as the Decline Conference, to understand why myocardial infarctions had suddenly decreased. Nearly 190 000 fewer deaths had occurred than expected. Antihypertensive medications were not yet widely used, and traditional risk factors did not fully explain the shift."’
