Sun & UV Exposure Planner日照防晒规划器
Skin type and SPF → safe minutes, reapply times and sunscreen amounts for altitude and snow. 皮肤分型与 SPF → 安全日晒分钟、补涂时间与高海拔雪地用量。
How the safe-minutes model works
Sunscreen timing is not magic — it starts from a measurable base. This planner combines your Fitzpatrick skin type, the labelled SPF, the day’s UV index, elevation and the reflective surface around you to estimate minutes until the first sunburn, then layers a practical reapplication schedule.
The reference point is midday summer sun at a UV index of 10. Published fair-skin data put unprotected time-to-burn at roughly 10, 15, 20, 25, 30 and 40 minutes for Fitzpatrick types I through VI. Because the UV index is a linear scale, time-to-burn scales inversely: at UVI 5 the same skin tolerates about twice as long, at UVI 11 somewhat less. Labelled SPF then multiplies that window — so a type-I hiker wearing SPF 30 at UVI 10 gets about 300 estimated minutes before redness, and type VI with SPF 15 gets about 600 minutes. In the field two corrections shrink those numbers. UV rises approximately 10% per 1000 metres of elevation because thinner air absorbs and scatters less of it, and reflected radiation comes at you from below: sand and water reflect 10–25% (factor 1.2), while fresh snow reflects up to 80% (factor 1.5). At 3000 m on snow, the type-III SPF 30 estimate falls from 600 to roughly 308 minutes.
Why the label and the trail disagree
SPF ratings are measured with 2 mg of product per square centimetre — a thick layer almost nobody applies. Real-world coverage of one-quarter to one-half of that amount can drop an SPF 30 product to an effective SPF near 10. That is why the calculator emphasises dose: 30 ml (about a shot glass) for an adult body in swimwear and roughly 5 ml (a teaspoon) for the face, neck and ears, plus an SPF 30 lip balm. Apply 15–30 minutes before exposure so the film can bind, and reapply every two hours at rest, roughly every 80 minutes while sweating, and every 40 minutes while swimming or towelling.
SPF has a ceiling; clothing does the heavy lifting
SPF 30 filters about 97% of UVB, SPF 50 about 98% and SPF 100 around 99% — never 100%. Above SPF 50 the marginal gain is small, which is why dermatologists prefer correct application of SPF 30–50 over a false sense of security from SPF 100. When the UV index is 8 or higher, seek shade between 10:00 and 16:00, wear tightly woven or UPF-rated clothing and a wide-brim hat, and wear UV400 sunglasses; on glaciers add side-shield glacier glasses. Infants under six months skip sunscreen entirely — shade and clothing only.
安全日晒分钟是怎么算出来的
防晒霜的有效时间并不神秘——它从一个可测量的基数出发。本规划器结合你的 Fitzpatrick 皮肤分型、标签 SPF、当天紫外线指数、海拔与周围反射表面，估算到初次晒伤的分钟数，再给出实用的补涂计划。
参考点是紫外线指数 10 的夏季正午阳光。已发表的白皙皮肤数据显示，Fitzpatrick I 到 VI 型皮肤不做防护的晒伤时间约为 10、15、20、25、30、40 分钟。紫外线指数是线性标度，晒伤时间与它成反比：UVI 5 时耐受时间约翻倍，UVI 11 时更短。标签 SPF 再乘以这个窗口——所以 I 型皮肤在 UVI 10 用 SPF 30，预计约 300 分钟后才会发红；VI 型皮肤用 SPF 15 约 600 分钟。在野外还要做两个折减：海拔每升高 1000 米，空气更稀薄、吸收散射更少，紫外线约增加 10%；反射辐射还会从下方照来——沙地和水面反射 10–25%（系数 1.2），新雪反射可达 80%（系数 1.5）。在海拔 3000 米雪地上，III 型皮肤 SPF 30 的安全时长就从 600 分钟降到约 308 分钟。
为什么标签时长和实际体感不一致
SPF 评级是按每平方厘米 2 毫克的厚涂量测得的，现实中几乎没人涂够。只涂到测试量的四分之一到一半，SPF 30 的实际防护可能只剩 SPF 10 左右。因此计算器特别强调用量：穿泳衣的成人全身每次约 30 毫升（约一烈酒杯），脸、颈、耳朵约 5 毫升（一茶匙），嘴唇另用 SPF 30 以上润唇膏。出门前 15–30 分钟涂抹使其成膜；休息时每 2 小时补涂，徒步出汗约每 80 分钟，游泳或毛巾擦拭后每 40 分钟补涂。
SPF 有天花板，遮挡主要靠衣物
SPF 30 阻挡约 97% 的 UVB，SPF 50 约 98%，SPF 100 约 99%——绝不是 100%。超过 SPF 50 后边际收益很小，所以皮肤科更推荐正确使用 SPF 30–50，而不是迷信 SPF 100 带来的虚假安全感。紫外线指数 ≥8 时，10:00–16:00 找阴凉、穿密织或 UPF 认证衣物、戴宽檐帽与 UV400 太阳镜；冰川环境再加带侧挡的冰川镜。6 个月以下婴儿完全不用防晒霜，只靠阴凉与衣物防护。