Is My Sunscreen Safe for Freshwater?

The reef-safe conversation ignores lakes, ponds, and rivers. That's a problem.

Everyone who's read a sunscreen label in the last five years has seen "reef-safe." It's become shorthand for environmentally responsible SPF. The problem: it only covers one ecosystem. And it may not be the most vulnerable one.

The ocean is enormous. Your lake is not.

The coral reef narrative makes sense as a starting point. Oxybenzone and octinoxate show up in ocean water near popular dive sites. The data is real. But the ocean is a massive, dynamic system. Currents move. Water circulates. The dilution factor is enormous.

A lake doesn't work that way. Neither does a pond, a river swimming hole, or a reservoir. These are closed or semi-closed systems with limited capacity to dilute, break down, or flush out what enters them. What goes in tends to stay in.

Three categories of compounds that accumulate

Most sunscreens contain three categories of ingredients that don't belong in freshwater systems.

The first is synthetic UV filters: oxybenzone, octinoxate, homosalate, octisalate. These are synthetic organic compounds that don't occur naturally in freshwater ecosystems. A geospatial study published in ACS ES&T Water detected benzophenone-3 (oxybenzone) directly in a fish-bearing river. A national-scale environmental risk assessment published in PMC found oxybenzone present in rivers, lakes, and groundwater across the United States, with a freshwater half-life estimated at 2.4 years. It dissolves easily in water and fat and does not readily biodegrade. In a closed system, it accumulates.

The second is silicones. Dimethicone, cyclopentasiloxane, and related compounds are standard ingredients in sunscreen formulas. They create the smooth spreadable texture most consumers associate with SPF. The problem is they are not biodegradable. They persist. The European Chemicals Agency has flagged cyclosiloxanes D4 and D5 as persistent, bioaccumulative, and toxic. Research published in ScienceDirect found D4, D5, and D6 accumulating in the sediment and water of Dian Lake, and an eight-year monitoring study of Lake Pepin in Minnesota documented cyclosiloxane bioaccumulation across the food web. In an ocean, persistent compounds at least have the possibility of dispersal. In a lake, they settle.

The third is polymers. Acrylates, carbomers, and similar polymer-based thickeners and film-formers appear throughout conventional sunscreen formulations. In water, synthetic polymers behave like microplastic precursors. A 2025 study in ACS Environmental Science and Technology specifically examined sunscreen-derived microplastics, finding that photodegradation of sunscreen polymers generates secondary microplastic particles that leach into surrounding water. Research confirms that freshwater sediments function as accumulation zones for these particles, where they are ingested by benthic organisms at the base of the food chain.

Who is most exposed

Freshwater organisms have no evolutionary framework for processing synthetic organic chemistry. A 2024 study in ScienceDirect documented developmental, behavioral, and biochemical effects on freshwater species from chronic sublethal exposure to organic UV filters. A review in PMC found endocrine disruption in frogs, fish, and aquatic invertebrates linked to UV filter exposure. Amphibians absorb compounds directly through permeable skin. Fish show reproductive hormone disruption. Aquatic invertebrates and insects that form the base of freshwater food chains accumulate silicones and polymers in their tissue.

As ecotoxicologist Craig Downs noted in a 2025 Mongabay investigation, these contaminants are found not only in coastal waters but in freshwater lakes and streams. The research also notes significant data gaps for freshwater amphibians and sediment organisms, which means the full picture is likely worse than current studies show.

Colorado is our home. These are our waters.

Swellies is based in Denver. We built this formula in Colorado and we spend time in Colorado water. That matters to us directly.

Boulder Reservoir is the Front Range's primary open-water swimming destination. Cherry Creek Reservoir draws swimmers from across the Denver metro every summer. Chatfield, Carter Lake, Boyd Lake, and Grand Lake see thousands of visitors annually. Dillon Reservoir sits at 9,017 feet in Summit County and feeds the Denver metro water supply. Bear Lake and the alpine water systems inside Rocky Mountain National Park are among the most ecologically sensitive freshwater environments in the country.

High-altitude lakes compound the problem. Cold temperatures slow the biological processes that break down organic compounds. Lower microbial diversity means less natural processing capacity. Smaller water volume means faster accumulation. A chemical that disperses in an ocean to near-zero concentration can build in an alpine lake to levels that affect the organisms living in it.

None of these water bodies have the advocacy infrastructure that coral reefs do. There are no high-profile bans, no campaign imagery, no certification logos. There is just the water, and what ends up in it.

Why nobody talks about freshwater

Coral reefs are photogenic. They have visible damage, documented bleaching events, and a decade of conservation advocacy behind them. Freshwater doesn't have that imagery. A reservoir outside Denver doesn't generate the same campaign material as a bleached reef in Hawaii.

It also doesn't have a regulatory hook yet. Hawaii and several other jurisdictions have moved to ban specific chemical filters to protect coral. No equivalent freshwater legislation exists. Brands have no external pressure to address it, and most don't.

What actually belongs in water

Zinc oxide is a mineral. It occurs naturally in aquatic environments. Non-nano zinc oxide sits on the surface of skin rather than absorbing systemically, and it doesn't behave like a synthetic organic compound in water. No silicones. No polymers. No persistent chemistry settling into lake sediment.

That's what Swellies uses. One active: non-nano zinc oxide at 21%. The other four ingredients are Coco Caprylate/Caprate, Dextrin Palmitate, Polyhydroxystearic Acid, and Iron Oxides. No oxybenzone. No cyclopentasiloxane. No acrylates. No synthetic chemistry that accumulates in a closed water system because there's nowhere else for it to go.

Reef-safe is accurate. But the cleaner answer for freshwater is the same formula: five ingredients, one mineral active, nothing persistent.

The short version

If you're swimming in Boulder Reservoir, floating on Dillon, or hiking into the backcountry lakes of Rocky Mountain National Park, the reef-safe label on your sunscreen doesn't tell you much. The question is whether your formula contains synthetic UV filters, silicones, or polymers that accumulate in closed water systems with no mechanism to clear them.

Most sunscreens do. Swellies doesn't.