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Donald Trump's Sniffling: What Causes Sniffles When You Don't Have a Cold?
Hillary Clinton and Donald Trump faced off in the first 2016 presidential debate at Hofstra University in New York on Sept. 26, 2016.
Credit: Drew Angerer/Getty Images

Many watchers of Monday night's presidential debate noted that Donald Trump kept sniffling. The presidential candidate later said that he didn't have a cold, so what else might have caused the sniffling?

During the debate, Trump's frequent sniffling was a widely discussed topic on social media, giving rise to the hashtag #Trumpsniffle on Twitter. In a TV interview with "Fox and Friends" the following morning, Trump denied being sick, saying "No, no sniffles, no cold," according to Reuters.

While we don't know the reason behind Trump's frequent sniffling, a number of things can cause the symptom.

"It could be almost anything," said Dr. Alan Mensch, a pulmonologist and senior vice president of medical affairs at Northwell Health’s Plainview and Syosset Hospitals in New York, who is not involved in Trump's medical care. [7 Absolutely Horrible Head Infections]

Anytime the mucous membranes in the nose react to something by swelling, it can cause people to sniffle, Mensch said.

This swelling can be triggered by allergies (such as hay fever), irritants in the air (such as cigarette smoke, perfume or dust), and a viral infection (even before you have full-blown symptoms).

People can also experience constant sniffling if they use a nasal spray to treat their allergies or cold symptoms. When the effect of the nasal spray wears off, people can sometimes experience a "rebound effect" in which their symptoms are even worse than before they used the medication, Mensch said.

 Although not applicable to Trump, Mensch said that being pregnant can also lead to an increase in sniffling. During pregnancy, mucous membranes swell throughout the body, including in the nose, he said.

Even cold weather can cause us to sniffle. This happens because our noses typically add moisture to the air we breathe, but if the air is cold and dry, the nose may increase fluid production, leading to a runny nose and sniffling, according to NPR.

When asked about his sniffling, Trump brought up a technical issue rather than a medical one. "The mike was very bad, but maybe it was good enough to hear breathing,” he told "Fox and Friends."

Original article on Live Science.