We may be more conscious of being surrounded by tiny droplets of water since we were caught up in a global pandemic. Droplets in moist air, or when coughing, sneezing or talking can carry tiny particles, from pollution to viruses such as the flu.

Back in 2019, the basic chemistry of these ambient water particles brought a surprise: Researchers found that ordinary benign water droplets can somehow spontaneously form small but significant amounts of hydrogen peroxide (H2O2).

Yes, the weak acid we use to bleach our hair or disinfect our wounds.

Now, a team of researchers has found that this spontaneous response occurs when touching solid surfaces, and it may play a role in flu seasonality.

“Water is one of the most common substances we thought we knew so much about, but then we were ashamed,” said Stanford University chemist Richard Zare, who was involved in both findings. .”

Volume-wise, water is fairly stable, but breaking it into tiny droplets, by contrast, seems to drastically alter its behavior.

As objects shrink, their relative volume and specific surface area decrease faster. This means that a tiny water droplet is exposed to a much larger proportion of molecules in its surroundings than it would in a glass, bucket or lake.

It was on this exposed surface that hydrogen peroxide formed, chemist Chen Bolei of Jianghan University and his colleagues confirmed.

Using a dye that glows in the presence of H2O2, the team mapped what happened in a droplet touching the glass surface and found that it was most concentrated at the interface of the two substances.

The researchers demonstrated that H2O2 is also produced when droplets come into contact with nine other solids, including soil or fine dust suspended in the air.

In a previous study, they also showed that this happens naturally, only when water condenses from the air onto a cold surface. The H2O2 content increases with increasing humidity.

“Contact electrification to produce hydrogen peroxide appears to be a common phenomenon at the water-solid interface,” explains Zare.

To figure out where the droplets stole the excess oxygen atoms, the researchers treated the glass surface with the heavy oxygen isotope 18O.

Sure enough, the droplets formed hydrogen peroxide by absorbing 18O atoms on the glass surface, confirming that the excitable elements of hydrogen and oxygen — hydroxyl radicals — attached to the surface material were the source of the hydrogen peroxide.

As the acid formed, Chen and his team could also measure the current flowing from the solid to the ground as the dye’s luminescence increased.

This confirms previous findings that electron exchange occurs in a process known as contact charging, producing hydroxyl radicals.

This of course does not rule out that some of the raw materials may come from other sources in the environment, such as ozone (O3), the team wrote in their paper. But it did confirm that only water droplets and solid surfaces were needed.

“Contact energization provides a chemical basis that partially explains why viral respiratory diseases are seasonal,” Zare said.

With increased moisture in the warm summer air, carrying small amounts of hydrogen peroxide, this could be another hurdle that circulating pathogens need to deal with.

Instead, cold, dry winter air may only give viruses a small advantage, allowing them to jump from sneezing nose to sneezing nose.

The research was published in the Proceedings of the National Academy of Sciences.