Fox News medical contributor Dr. Marc Siegel told "Tucker Carlson Tonight" there is "no science" behind former Vice President Joe Biden's call Thursday for a nationwide mask mandate to help fight the spread of the coronavirus pandemic.

Host Tucker Carlson called it one of the presumptive Democratic presidential nominee's first campaign promises: a three-month mandate requiring all Americans to wear masks, including outside.

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“Every American should be wearing a mask when they’re outside for the next three months at a minimum,” Biden told reporters in Wilmington, Del., estimating it will save at least 40,000 lives and emphasizing, "It's not about your rights, it's about your responsibilities as an American."

Democratic presidential candidate former Vice President Joe Biden and his running mate Sen. Kamala Harris, D-Calif., talk to media during a virtual briefing on COVID-19 from public health experts in Wilmington, Del., Thursday, Aug. 13, 2020. (AP Photo/Carolyn Kaster)

Siegel called it "the politics of fear" pointing out that recent studies have shown that wearing masks "under proper conditions, when you're close together or your indoors and the ventilation is poor that they likely decrease the viral spread."

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"Now that's not proof, it's enough for me to like the idea of masks," the doctor said, calling it the "politics of fear, big brother edition," questioning the scientific basis for having Americans wear masks even when they are outdoors and can socially distance from others.

Biden did not take questions after sitting down for a briefing on COVID-19 with public health experts, along with his newly named running mate, Sen. Kamala Harris of California.

Fox News medical contributor Dr. Nicole Saphier told "Fox & Friends" Friday that Biden needs to provide more details when he says masks should be worn outdoors at all times.

"Because the last I saw, the transmission level for this virus is significantly less outdoors," she said, adding that "the current recommendations are to wear a mask if you're not able to socially distance not just at all times."

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Saphier questioned if Biden wants restaurants to close outdoor dining once again, despite no evidence showing it causes the spread of COVID-19.

President Trump slammed Biden for attempting "to politicize a pandemic" arguing Biden’s approach was “regressive, unscientific and bad for our country.”