Vehicle exhaust includes a variety of particulates from burning fuel, including carbon nanotubes. Nanotubes are worrisome because their long, thin shape resembles much larger micrometer-scale asbestos fibers, which can linger in lung tissue and are known to cause respiratory diseases. A new study has shown that sulfur and ferrocene found in diesel fuel promote the formation of roughly 100-nm-long nanotubes in a car engine (Environ. Sci. Technol. Lett. 2016, DOI: 10.1021/acs.estlett.6b00313). Jacob J. Swanson, a mechanical engineer at Minnesota State University, Mankato, and his colleagues systematically introduced fuel with increasing concentrations of sulfur and ferrocene into a diesel engine in their laboratory. They captured the exhaust and examined the shape of the emitted particulates using transmission electron microscopy. When the fuel had only sulfur or only ferrocene, a few nanotubes appeared. But at the highest concentrations tested when both components were present, 4,500 ppm sulfur and 36 ppm ferrocene, the researchers saw an appreciable increase in the number of nanotubes in the images collected. This sulfur concentration is comparable to levels in diesel fuel in some countries, though higher than in the U.S. and other developed countries, meaning exposure to nanotubes could potentially be greater in places with less stringent fuel standards, the researchers say.