Polluted seawater worsens inland air quality, study finds

Contaminants such as bacteria, viruses and chemical compounds in polluted coastal waters transfer to the atmosphere in sea spray aerosol formed by breaking waves and bursting bubbles.

Researchers from Scripps Institution of Oceanography at UC San Diego studied the impact of untreated sewage being diverted into the Tijuana River, which lies on the US-Mexico border, and flows into the ocean in south Imperial Beach in San Diego.

“We’ve shown that up to three-quarters of the bacteria that you breathe in at Imperial Beach are coming from aerosolisation of raw sewage in the surf zone,” said lead researcher Kim Prather.

“Coastal water pollution has been traditionally considered just a waterborne problem. People worry about swimming and surfing in it but not about breathing it in, even though the aerosols can travel long distances and expose many more people than those just at the beach or in the water.”

The team sampled coastal aerosols at Imperial Beach and water from the Tijuana River between January and May 2019.

They then used DNA sequencing and mass spectrometry to link bacteria and chemical compounds in coastal aerosol back to the sewage-polluted Tijuana River flowing into coastal waters.

The researchers said their work does not necessarily mean people are getting sick from sewage in sea spray aerosol. Most bacteria and viruses are harmless and the presence of bacteria in sea spray aerosol does not automatically mean that microbes – pathogenic or otherwise – become airborne.

Infectivity, exposure levels and other factors that determine risk need further investigation, the authors added.

“This research demonstrates that coastal communities are exposed to coastal water pollution even without entering polluted waters,” said lead author Matthew Pendergraft.

“More research is necessary to determine the level of risk posed to the public by aerosolised coastal water pollution. These findings provide further justification for prioritising cleaning up coastal waters.”

Last summer, heavy storms hit the UK after months of little rain which overwhelmed sewage systems. Over 40 beaches and swimming pools were consequently subject to pollution warnings.