Bans on certain sunscreen ingredients in reef destinations have made the subject familiar without making the mechanism clear. The pathway from a bottle to a coral colony has two distinct routes.
Direct washing off is the obvious route
A proportion of applied sunscreen washes off within the first minutes of immersion, which is why reapplication after swimming is standard advice.
That material enters the water immediately around the swimmer, so concentration depends on how many people are in a given volume of water.
Sheltered lagoons and enclosed bays are the problem locations, because water exchange is limited and the same bodies of water hold bathers all day.
Wastewater is the larger and less visible route
Sunscreen washed off in showers enters the sewage system, and conventional treatment does not fully remove these compounds.
Treated effluent discharged near the coast therefore carries a continuous low-level load, which reaches reefs regardless of whether anyone swam there.
In destinations with limited treatment infrastructure this route can exceed the direct one, which is why bans alone do not resolve the issue.
The concern is developmental rather than acute
Laboratory work on certain ultraviolet-filtering compounds has found effects on coral larvae and on the algae that corals depend on, at low concentrations.
The effects reported involve settlement, development and bleaching susceptibility rather than immediate mortality of adult colonies.
How closely laboratory concentrations correspond to conditions on a real reef remains debated, which is the main reason the scientific picture is less settled than the regulatory one.
Mineral alternatives shift the question
Mineral filters based on zinc or titanium sit on the skin and reflect or absorb ultraviolet rather than being absorbed into it.
Their environmental behaviour depends heavily on particle size and coating, and very fine uncoated particles behave differently from larger ones.
Products labelled as reef safe are not certified against an agreed standard, so the term describes an intention rather than a verified property.
Coverage removes the exposure entirely
A long-sleeved swim shirt eliminates the need for sunscreen over the largest area of skin, which reduces both the quantity applied and the quantity washed off.
It also provides more reliable sun protection than a coating that thins with time and immersion, which is a practical benefit independent of the reef question.
Applying remaining sunscreen well before entering the water allows it to bind to skin, which reduces the fraction that rinses off in the first minutes.