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Solar storms that disrupt Earth鈥檚 magnetic field may cause up to 5500 heart-related deaths in the US in some years.

The sun goes through cycles of high and low activity that repeat approximately every 11 years. During periods of high activity, it blasts out charged particles and magnetised plasma that can distort Earth鈥檚 magnetic field.

These so-called solar storms can cause . A handful of studies have also hinted that they increase the risk of heart attacks, but these were too small to be conclusive.

To explore further, at Harvard University and her colleagues analysed records of deaths between 1985 and 2013 in 263 US cities. They then compared heart-related fatalities with solar storm data.

They found that more heart disease deaths occurred on days when solar storms had disturbed Earth鈥檚 magnetic field.

For each year of high solar activity during that period, they estimate that an additional 5500 people in the US died of heart attacks or other cardiovascular complications.

To better understand the mechanisms behind this phenomenon, Zilli Vieira and her colleagues analysed electrocardiograms (ECGs) 鈥� tests that monitor the heart鈥檚 electrical activity 鈥� that were conducted on more than 800 men in the US as part of a long-term health study.

They discovered the men were more likely to display reduced heart rate variability, a sign their hearts were stressed, when solar storms had disrupted Earth鈥檚 magnetic field in the past 24 hours. This association was particularly strong for the men with diagnosed heart disease.

Referring to the results, at the Australian National University in Canberra , says: 鈥淚t sounds left field, but they have done a robust analysis.

鈥淲hen we think of cardiovascular risk factors, we tend to think of things like high blood pressure, diabetes and smoking, but this suggests we need to pay more attention to the impacts of the wider environment.鈥�

Solar storms shift Earth鈥檚 magnetic field, disrupting normal circadian rhythms, also known as our body clocks. These play an important role in regulating our heart rate and other bodily functions, says Zilli Vieira.

Higher solar activity has also been associated with increased rates of and , which may be similarly linked to circadian disruptions, she says.

Other species appear to be affected too, with and during solar storms.

鈥淚f you think about how the sun makes up over 99 per cent of the mass of our solar system, it makes sense that it could affect our health in various ways,鈥� says Zilli Vieira.

Her team is now trying to understand exactly how solar activity affects the heart so they can find ways to protect vulnerable people when solar storms are predicted. The next big surge in solar activity is .

鈥淚 do believe that people should know about this, and in the future, we can hopefully develop some mitigation measures,鈥� says Zilli Vieira.