Philadelphia's live UV index is shown at the top of this page and updates every 30 minutes. Reading it: 0-2 is low (no protection needed for most people), 3-5 is moderate (sunscreen for long stretches outside), 6-7 is high (SPF 30+, shade at midday), 8-10 is very high (reapply every 2 hours, shade between 10am and 4pm), and 11-plus is extreme. Lighter skin (Fitzpatrick I-II) burns at lower tiers and after shorter exposures. The vitamin D trade-off is smaller than most people think: brief incidental exposure most days is usually enough.
Today's UV reading for Philadelphia is at the top of this page and refreshes every 30 minutes, so the number you see is the number outside right now. What you do with it depends on the tier, your skin type, and how long you will be outside. For skin health, the UV index is the clearest single number to manage, and one of the most ignored. Most adults who come in with sun damage did not have one bad week; they had a slow accumulation of moderate-UV days over decades.
What the UV Index measures
The UV index is a forecast of the strength of ultraviolet radiation reaching the ground, calibrated to skin damage. It runs from 1 (very low risk in a few minutes) to 11-plus (extreme; damage can occur in minutes in unprotected skin).
UVB (280-315 nm) is the shorter-wavelength UV that drives sunburn. It penetrates the epidermis (the outer layer of skin), where it damages DNA in keratinocytes and melanocytes. It is responsible for most of the skin cancer signal, including basal cell carcinoma, squamous cell carcinoma, and a substantial portion of melanoma. It is the same wavelength that drives vitamin D synthesis in the skin.
UVA (315-400 nm) is the longer-wavelength UV that penetrates more deeply, into the dermis. It is the main driver of photoaging, dermal collagen breakdown, hyperpigmentation, and elastosis. It also contributes to skin cancer, and unlike UVB, it passes through window glass, which is why long drives and window seats matter for skin and eye exposure.
Any single reading matters less than the cumulative dose your skin has logged. The UV index is the proxy for the dose you are getting today.
How skin type changes the math
Sun damage does not affect every skin type the same way. The Fitzpatrick scale classifies skin from I (always burns, never tans, fair and freckled) to VI (deeply pigmented, almost never burns). Higher melanin content offers some natural UV defense, but it does not eliminate skin cancer risk; melanomas in darker-skinned people are often diagnosed later and on less-sun-exposed sites (palms, soles, under nails), making outcomes worse.
- Fitzpatrick I-II (fair, often burn): the action tier for sun protection starts one step earlier than the chart suggests. A "moderate" day is your "high" day.
- Fitzpatrick III-IV (medium, sometimes burn, tan): the chart guidance applies directly.
- Fitzpatrick V-VI (darker, rarely burn): the burn risk is lower but skin cancer risk is not zero; the bigger long-term issues for many are hyperpigmentation, melasma, and overlooked melanomas on non-sun-exposed sites.
If you have a personal history of melanoma, nonmelanoma skin cancer, immunosuppression, or you are on a photosensitizing medication (tetracyclines, certain diuretics, retinoids, methotrexate), treat your protection as one tier higher across the board.
The vitamin D trade-off
UVB is the wavelength that converts 7-dehydrocholesterol in your skin into previtamin D3. The reaction needs UVB above about index 3, which in Philadelphia means roughly April through September during midday hours. From October through March at our latitude, UVB is too weak at any time of day to produce meaningful vitamin D, no matter how long you stand outside.
The good news is that the dose-response curve plateaus quickly. Brief incidental exposure of arms and lower legs for about 10 to 20 minutes around midday in summer for fair skin, or 25 to 40 minutes for darker skin, produces a meaningful amount of vitamin D. Past that, the vitamin D yield levels off while the skin damage continues to accumulate. There is no benefit to spending an hour unprotected in midday sun "for the vitamin D."
The clear clinical answer: get brief casual exposure in the warm months as a baseline, supplement vitamin D in the cold months (and in the warm months if your levels are low), and use sun protection for everything else. See our vitamin D3 + K2 clinical guide for dosing logic.
Low (UV Index 0-2)
Minimal danger from sun exposure for the average person.
- Most people: routine sun protection is not required. Sunglasses are still helpful for eye comfort. A morning walk on the Schuylkill River Trail is a no-brainer.
- Fair skin (Fitzpatrick I-II): if you will be outside for more than an hour, broad-spectrum SPF 30 on face, ears, neck, and hands is still worth applying.
- History of skin cancer or immunosuppression: treat as moderate; sunscreen on exposed skin even at this tier.
- Vitamin D note: UV index this low does not produce meaningful vitamin D, regardless of season.
Low-UV months are when vitamin D slips, and Philadelphia has a lot of them. If yours has not been checked in the last year, tell Dr. Ash what you're working on and it goes on your first lab panel.
Moderate (UV Index 3-5)
Moderate risk of harm from unprotected sun exposure. Most people can be outside safely for an hour with reasonable protection.
- Most people: broad-spectrum SPF 30 on exposed skin if you will be outside more than 30 minutes. Sunglasses with UV protection. Wide-brim hat for prolonged exposure. Reapply sunscreen every 2 hours and after sweating heavily.
- Fair skin (I-II): treat as high; reapply sunscreen on a stricter schedule.
- Kids: sunscreen and a hat for any prolonged outdoor time. Toddlers under 6 months should be in shade rather than wearing sunscreen.
- Vitamin D note: brief midday exposure of arms and lower legs (10 to 20 minutes for fair skin) produces useful vitamin D at this tier in summer.
Moderate days are where decades of sun damage add up, one "this is fine" afternoon at a time. If you carry a risk factor and have never had a skin-risk review, start your intake and Dr. Ash will fold one into your preventive plan.
High (UV Index 6-7)
High risk of harm from unprotected exposure. Protection against skin and eye damage is needed.
- Most people: broad-spectrum SPF 30 to 50 on all exposed skin. Reapply every 2 hours and after swimming or sweating. UV-blocking sunglasses. Wide-brim hat. Seek shade between 10 am and 4 pm.
- Long outdoor sessions: UPF clothing (sun shirts, long sleeves, hiking pants) outperforms repeated sunscreen reapplication for arms and legs. The Schuylkill Trail and Wissahickon offer limited shade; plan for it.
- Jersey Shore caveat: sand and water reflect 10 to 25 percent of UV upward, so the effective UV exposure under a beach umbrella can equal a partly-protected dose. Treat shore days as one tier higher.
- Fair skin (I-II), kids, and people with a personal history of skin cancer: UPF clothing plus sunscreen, not either alone.
Outside a lot for training, work, or the trail? Preventive care here keeps the sun math managed for you: the yearly skin check on the calendar, the vitamin D level on the panel, and a doctor you can text a photo of any spot that changes.
Very High (UV Index 8-10)
Longevity Medicine
A personalized longevity strategy starts with knowing your real baselines.
Very high risk. Damage can occur quickly in unprotected skin.
- Everyone: Avoid the sun between 10 am and 4 pm as much as possible. Broad-spectrum SPF 50, reapplied every 90 minutes if you are outside. UPF 50 long-sleeved shirts. Wide-brim hat. Wraparound UV-blocking sunglasses (eye damage at this tier accumulates fast). Mineral sunscreens (zinc oxide, titanium dioxide) are the easiest broad-spectrum option for sensitive skin.
- Kids: outdoor time before 10 am or after 4 pm. If they are at camp or sports practice midday, hats and sunscreen are mandatory, not optional. Pre-cooled water and frequent shade breaks; sun and heat tend to overlap at this tier.
- Eagles tailgates, Phillies day games, marathons: plan UPF and sunscreen the same way you plan hydration. Reapply at every break.
- Fair skin (I-II), kids, and high-risk patients: any unprotected midday time at this tier can cause damage. There is no responsible "I'll just dash out for 20 minutes."
A burn that blisters, or a new rash after a day in the sun, deserves a look. Members send Dr. Ash a photo and hear back the same day. If that is the kind of care you want on days like this, start your intake.
Extreme (UV Index 11-plus)
Extreme risk. Damage can occur within minutes in unprotected skin.
- Everyone: Avoid all midday sun. Schedule outdoor activities for early morning or evening. If you must be out, full UPF coverage (long sleeves, pants, wide brim, wraparound sunglasses) plus SPF 50 on any exposed skin, reapplied every 60 to 90 minutes.
- Heat stress overlaps with extreme UV. Hydrate with water and electrolytes, take frequent shade breaks, watch for the early signs of heat exhaustion (cramps, headache, lightheadedness, nausea). At this tier, both systems are stressed at once. On a declared heat health emergency, cooling centers and the Heatline are there too.
- People on photosensitizing medications: even brief exposure can produce a phototoxic reaction. Confirm with your prescriber whether to time your dose differently.
- Eyes: UV at this tier accelerates cataract and macular degeneration risk over years. Wear quality wraparound sunglasses with full UV protection (look for "UV 400" labels).
Not sure whether one of your medications makes you photosensitive? Thats a 2-minute question when your doctor answers texts. Tell Dr. Ash what's going on.
Sunscreen
Most sunscreen "controversies" online are about the wrong things. For the deeper playbook - how much to use, UPF clothing, reef-safe options, treating a burn, and when to get a spot checked - see our sun protection guide.
- SPF 30 broad-spectrum is enough for daily use. SPF 50 is the right move on high-UV days, water exposure, and long outdoor sessions.
- SPF 100 is not much stronger than SPF 50. The scale is logarithmic: SPF 50 filters roughly 98 percent of UVB, and SPF 100 filters about 99. The bigger number buys you one extra percentage point, and it does not add a single minute to the reapplication clock. Buy whichever texture you will rewear, and reapply on the same schedule either way.
- Broad-spectrum means the sunscreen also blocks UVA, which drives photoaging and adds to cancer risk. The SPF number by itself does not measure UVA.
- Mineral sunscreens (zinc oxide, titanium dioxide) are the easiest broad-spectrum option for sensitive skin and pregnancy. They sit on the skin and reflect UV. Modern formulations are cosmetically much better than the white-cast versions older patients remember.
- Chemical sunscreens (avobenzone, octinoxate, oxybenzone) absorb UV and convert it to heat. They are well-studied and effective; the systemic absorption studies that made headlines did not find clinical harm. If you want to skip oxybenzone for reef-friendliness or personal preference, plenty of effective options exist without it.
- Reapplication is the variable that matters most. Most people apply too thin and reapply too late. The standard adult facial dose is about a quarter teaspoon; for the body, about an ounce. Reapply every two hours, and after swimming, heavy sweating, or toweling off.
- On Very High and Extreme days, compress the clock. The two-hour rhythm is calibrated to ordinary days. When the index is 8-plus - or any beach day - move to roughly every 60 to 90 minutes while you are actively outside, and immediately after water. Doubling or tripling the cadence on the advanced days does far more for you than doubling the SPF number ever will.
A short word on the Jersey Shore
Cape May, Ocean City, Stone Harbor, Long Beach Island. Half of Philadelphia is at the shore on summer weekends.
- Reflection effectively bumps the UV tier up. Sand reflects roughly 15 to 25 percent, water 10 to 25 percent. The dose under an umbrella on the beach is materially higher than the same UV index in a city park. The practical translation: whatever the app says at the shore, read it one tier higher and act accordingly.
- Cloud cover does not save you. 70 to 90 percent of UV passes through light cloud cover, sometimes more under high thin clouds. A "cloudy" beach day can produce a worse burn than a bright one because people skip protection.
- Reapplication is the failure mode. Sweat, water, and sunscreen all interfere with each other on long beach days. Set a timer.
Guidance from the clinic
Actionable Steps
Three concrete moves to set up this week.
- Bookmark a live UV source. Open your weather app and confirm it shows UV Index; if it does not, install one that does. We surface the current Philadelphia UV Index on our homepage when it is high.
- Stock the kit. One broad-spectrum SPF 30 for daily face use, one SPF 50 for outdoor days, one UPF 50 long-sleeve shirt, one wide-brim hat, one pair of UV-blocking wraparound sunglasses.
- Schedule the basics. Yearly full-body skin check with dermatology if you have any risk factor (fair skin, family history of melanoma, prior skin cancer, immunosuppression, heavy sun history). Check vitamin D level once a year if not already on a stable protocol.
Key Takeaways
- The UV Index runs 1 to 11-plus and is a calibrated forecast of how fast UV can damage your skin.
- UVB drives sunburn and most skin cancer; UVA drives photoaging and also contributes to cancer. Both are amplified by water, sand, and concrete reflection.
- Fair skin (Fitzpatrick I-II), personal skin cancer history, immunosuppression, or photosensitizing medications all bump your effective tier up by one.
- The vitamin D trade-off plateaus fast; 10 to 20 minutes of midday summer exposure for fair skin is usually plenty, and supplementation handles the winter months.
- The protective moves that matter at higher tiers: broad-spectrum SPF reapplied on a clock, UPF clothing, midday shade, and quality UV-blocking sunglasses.
Scientific References
- Whiteman, D. C., Whiteman, C. A., & Green, A. C. (2001). Childhood sun exposure as a risk factor for melanoma: a systematic review of epidemiologic studies. Cancer Causes & Control, 12(1), 69-82.
- Holick, M. F. (2007). Vitamin D deficiency. New England Journal of Medicine, 357(3), 266-281.
- Lim, H. W., Arellano-Mendoza, M. I., & Stengel, F. (2017). Current challenges in photoprotection. Journal of the American Academy of Dermatology, 76(3S1), S91-S99.
- Sander, M., Sander, M., Burbidge, T., & Beecker, J. (2020). The efficacy and safety of sunscreen use for the prevention of skin cancer. Canadian Medical Association Journal, 192(50), E1802-E1808.
- International Agency for Research on Cancer. (2009). A review of human carcinogens. Part D: radiation. IARC Monographs on the Evaluation of Carcinogenic Risks to Humans, 100D.
- World Health Organization. (2002). Global solar UV index: A practical guide. Geneva: WHO.
Related at Fishtown Medicine
- Philadelphia AQI: what to do - the practical playbook for air quality days in Philly
- Pollen in Philadelphia - the tree/grass/weed calendar and when to pre-treat
- Microplastics in Philadelphia - the five highest-leverage moves to lower your exposure
- Severe Weather in Philadelphia - heat, cold, and storm preparedness for chronic disease patients
- Sun protection for Philly and the Jersey Shore - the full playbook on sunscreen, UPF clothing, and treating a burn
- Hydration and electrolytes - why water alone does not hydrate, and what to do on hot days
- Philadelphia Environmental Health (overview) - the city-wide environmental health framework
Frequently Asked Questions
Common Questions
Deep-Dive Questions
Ready when you are
Dr. Ash reads every intake himself, and answers questions personally - usually within a few hours.
