As a reminder, the main problem with gas-powered two-stroke engines is not the most obvious one: that they are so loud. Rather it is that the engines are so disproportionately dirty, emissions-heavy, and polluting. That's why they have been outmoded or outlawed for most other uses in Europe and North America, except for leaf blowers and other lawn equipment.
Over the past half-century, emissions from car and truck engines have gone down by 95% or more (depending on the measure you use). Power-generation is rapidly shifting toward renewable and less polluting sources. But the antique technology of two-stroke engines, which very inefficiently burn a mixture of oil and gas, remains an exception. Developing-world metropolises like Bangkok, Jakarta, Manila, and New Delhi have mounted major efforts to get two-stroke engines off their roads, because they contribute so heavily to pollution. (For more on the third-world aspects of this drama, see a list of references at the bottom of this item.) Fifty years ago, these engines were common in a variety of transport uses in the U.S. and Europe. These days they survive mainly in lawn equipment.
And now comes a report by Emily Underwood in Science magazine, whose subhead conveys its message: "Evidence builds that dirty air causes Alzheimer’s, dementia." That is, fine-particulate pollution, of which there are many sources but that two-stroke engines create to a disproportionate degree, is associated with mental problems in addition to its other known health effects. Sample:
Some of the health risks of inhaling fine and ultrafine particles are well-established, such as asthma, lung cancer, and, most recently, heart disease. But a growing body of evidence suggests that exposure can also harm the brain, accelerating cognitive aging, and may even increase risk of Alzheimer’s disease and other forms of dementia.
Please read the full story for more. It explains some of the fascinating biological and neurological hypotheses about how the mental damage might occur. Again the main point is: these engines are an outlier exception to the pace of cleaner and safer equipment in most other uses. And in much of the United States, the people are most at risk from long-term exposure to their effects are the hired lawn crews -- who are generally low-wage and not protected by long-term health insurance.
The Science story concludes, quoting a USC neuroscientist named Caleb Finch:
If PM2.5 is guilty as charged, they say, the goal for policymakers worldwide should be to push down levels as far as possible. When all the research is in, Finch says, “I think [air pollution] will turn out to be just the same as tobacco—there’s no safe threshold.”
More on what the rest of the world is doing about two-stroke engines:
- A World Bank study on the two-stroke engine problem, focusing on India: http://siteresources.worldbank.org/INTURBANTRANSPORT/Resources/2str1201improving.pdf
- A story from an Indian newspaper on banning two-stroke engines: http://www.thehindu.com/news/cities/bangalore/twostroke-engines-older-vehicles-come-under-scanner/article8318390.ece
- A story about bans in the Philippines: http://discovermagazine.com/2008/may/21-two-strokes-and-youre-out
- A Scientific American report in efforts across Asia: https://www.scientificamerican.com/article/better-engines-for-rickshaws/
- A study on how two-stroke scooters are more polluting than big trucks: http://www.telegraph.co.uk/news/science/science-news/10834679/Scooters-pollute-more-than-lorries.html
- A story from Sri Lanka: http://www.sundaytimes.lk/030406/ft/4.html
- A Nature magazine story with the headline, "Two Stroke Scooters are Dominant Source of Air Pollution in Many Cities," http://www.nature.com/articles/ncomms4749