Scientists have discovered a nanoparticle produced by jet engines that could be endangering human health. The pollutant reminds some researchers of an onion.

An international team of investigators discovered “onion-like” particles in aircraft jet exhaust while testing engines in Switzerland. Experts have long known that soot particles are produced by aircraft, typically found in higher concentrations closer to the engine.

But more than 15 meters from the engine, or just under 50 feet, about 99% of exhaust is composed of these never-before-seen particles. That means they’re a greater respiratory hazard for neighborhoods around airports.

Why does the study compare the particles to onions? That’s what they look like under an electron microscope. Investigators say they have concentric circular internal structure, resembling the layers of an onion.

And they’re small — 10 to 20 nanometers. That’s the size of a molecule or virus. So they can become deeply embedded in the lungs and potentially pass into the bloodstream.

The study says the particles appear to be created by a jet engine’s lubrication oil as it evaporates and decomposes in high temperatures. Their concentration varies with the type of engine.

It’s all cause for concern when it comes to public health policy and airport management, especially with the global growth of air travel. We already know that those living near airports have a higher risk of respiratory problems.

Even so, the health impact of the new particles is yet to be studied. Scientists, however, believe they might already contribute to health ills.

Pollution, after all, is rarely, if ever, harmless.