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How much sun exposure is needed to make vitamin D?

By Keith Roach, M.D. on

DEAR DR. ROACH: Does our skin make vitamin D from sun exposure through a car's windshield? Sometimes I expose my stomach for about 10 minutes while driving or riding in a car, but I'm not sure if the skin responds the same way through glass as it does with direct sun exposure. -- S.R.

ANSWER: Sunlight allows our skin to convert 7-dehydrocholesterol (made from cholesterol) to previtamin D3, which then becomes vitamin D3, the active form. The type of sunlight that is effective for this is in the ultraviolet spectrum, called UV-B. UV-B is effectively filtered out by windshields and ordinary glass, so you are NOT able to get vitamin D through the window: You need to go out into the sun. Ten minutes or so of exposure to your face and arms is roughly equivalent to 200 IU of vitamin D supplement. The exact amount depends on your skin type (lighter skin is much more efficient at using sunlight to convert vitamin D), your latitude, the season and the time of day.

UV-A light can get through most windows and light cloud cover, but not through UV-blocking window films that you can buy to block UV for your home and car windows. It does not help you make vitamin D. Unfortunately, it does penetrate deep into the skin, prematurely aging the skin and predisposing you to cancer. So, I wouldn't recommend exposing your skin through a windshield, as it won't cause health benefits and could cause health harms.

DEAR DR. ROACH: I had my annual flu shot a week ago. I am eligible for the Pfizer booster shot now. How long should I wait to get the booster shot? I am 82 years old, and my second COVID-19 shot was in mid-March. -- M.R.

ANSWER: Back in March, when you got the initial two doses, the Centers for Disease Control and Prevention and others recommended against getting any kind of vaccine within two weeks of getting the COVID-19 vaccine. That recommendation has been removed, and you can get the flu vaccine on the same day of your COVID-19 booster if you wish.

I do recommend the flu shot, which is available for all ages over 6 months. Last year, there was hardly any flu, because of the mask-wearing and social distancing protocols in place. These are still in place in many locations, so it's possible we could have another light flu season. However, they are NOT in place in others, and because children are back at school in most parts of the North America, there is likely to be a lot more flu than last year. Further, because essentially nobody got the flu last year, there is much lower resistance in the population than normal, so experts are worried about a severe flu season.

 

The flu makes you feel terrible, and with COVID-19 still out there, a person with the flu might legitimately worry about having COVID, necessitating quarantine and testing. It's far better to avoid this if you can by getting the flu shot and the COVID-19 booster if you meet the guidelines. It is certainly true that the flu vaccine isn't perfect, but it remains the best way to prevent flu.

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Dr. Roach regrets that he is unable to answer individual letters, but will incorporate them in the column whenever possible. Readers may email questions to ToYourGoodHealth@med.cornell.edu or send mail to 628 Virginia Dr., Orlando, FL 32803.

(c) 2021 North America Syndicate Inc.

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