How does SPF really work?
My #1 advice to my patients for skin longevity is to use SPF. But what does SPF actually mean, and how does it protect your skin?
What Is SPF?
SPF means Sun Protection Factor. It measures how well a sunscreen can shield your skin from UVB rays which is the type of ultraviolet radiation responsible for sunburn and a major contributor to skin cancer.
This means: if your skin would normally begin to burn after ten minutes in the sun, an SPF 30 product would theoretically allow you to stay in the sun 30 times longer without burning. That’s up to 300 minutes. However, it's not always like that. In real life, factors like sweating, how much you apply, and the strength of the sun all affect how long your sunscreen remains effective.
UVB vs UVA Rays
You often see this on your cosmetics so it’s important to understand the difference between UVB and UVA rays. UVA penetrates deep into the skin and contributes to aging and long-term damage and UVB affects the outer layers and causes sunburn. While SPF specifically measures protection against UVB rays, UVA rays are just as important to consider. These UVA rays penetrate more deeply into the skin, accelerating aging and increasing the risk of skin damage over time.
To guard against both types of rays, you need to use a product labeled "broad-spectrum." This means it offers protection from both UVB and UVA radiation, not just the burning rays but the aging ones as well.
How Sunscreens Work
Sunscreens use either chemical or mineral filters to block UV radiation.
Mineral filters, such as zinc oxide and titanium dioxide, sit on the surface of the skin and physically deflect UV rays. They’re effective immediately upon application and are often recommended for sensitive skin.
Chemical filters, like avobenzone or octocrylene, work by absorbing UV rays and converting them into heat. These require about 15 to 20 minutes to become fully effective after application. They’re often preferred for their lightweight, invisible finish, particularly under makeup.
Both types of filters can be effective when used correctly. Choosing the right one depends on your skin type, lifestyle, and personal preference.
Top Tips
Make sure you apply enough of the sunscreen to cover the desired protected area (and don't forget about your neck!). Reapplication is also key, especially if you’re outdoors, sweating, or in water. Every two hours is a good general rule.
It’s worth noting that products like SPF makeup or powders can be helpful for reapplication, but they shouldn't be your only source of sun protection.
SPF 30 vs SPF 50: Is There a Big Difference?
There is, but it’s more subtle than you might expect. SPF 30 blocks about 97 percent of UVB rays. SPF 50 blocks about 98 percent. While the difference in percentages seems small, in practice, especially for people with very fair skin, conditions like melasma, or a history of skin cancer, that extra protection can be meaningful.
The key takeaway is that no sunscreen blocks 100 percent of UV rays. Higher SPF can help, but consistency and proper use matter more.
Why You Still Need SPF on Cloudy Days and Indoors
UV radiation isn’t just a summer or beach-day issue. Up to 80 percent of UV rays can pass through clouds, and UVA rays can even penetrate glass. That means your skin is still vulnerable while driving, sitting by a window, or walking to work on an overcast morning.
If you struggle with hyperpigmentation or have skin prone to premature aging, consistent SPF use can make a noticeable difference over time.
What Matters
To get the full benefits of your sunscreen, choose a broad-spectrum product with an SPF of at least 30, apply enough of it, and reapply regularly. The best sunscreen is one you enjoy using every single day.
Sunscreen is more than a seasonal product. It’s one of the most effective tools we have to protect the skin against premature aging and serious long-term damage. When used correctly, it makes ALL the difference.