In The Early 20th Century What Heavily Polluting Substance Was Used For Mosquito Control?
Answer: Petroleum Oil
Although it’s unfathomable to a modern reader, given what we now know about environmental pollution and the impact we have on the environment, in early 20th century America a common method of mosquito control was the use of oil slicks.
In cities and on farms alike, people would pour petroleum oil into the water to create oil slicks which would in turn suffocate the mosquito larva. At the time, the long term environmental impact of pumping gallons of oil into ponds, marshes, and other water ways was poorly understood.
Although the use of petroleum-based oils in such a fashion is clearly horrible for the environment, there is a modern derivative of the same technique used on a smaller scale: people with rain catchment systems in mosquito-heavy areas will often float a thin layer of environmentally-safe vegetable oil on top of their collection container to kill off any mosquito larva found therein.
Image courtesy of the Broad Art Museum/Michigan State University.