How to Wash a Swimsuit

The Chemistry

Swimsuits are typically 78–82% polyester or nylon and 18–22% elastane (spandex/Lycra). Each component degrades from different environmental exposures, and understanding the mechanism explains why the rinsing step matters more than the washing step. Chlorine damage to elastane works via dehydrochlorination: hypochlorous acid (the active form of chlorine in pool water, formed when chlorine dissolves) attacks the polyurethane backbone of elastane molecules. Elastane is a polyurethane-polyurea copolymer — a chain of alternating hard and soft segments. Chlorine cleaves the urethane linkages through an oxidative mechanism, breaking the long polymer chain into shorter fragments. Shorter chains mean lower molecular weight, reduced elastic memory, and permanent loss of stretch recovery. The fibre cannot be repaired — once the polymer chains are broken, the elastane in that section of fabric has permanently reduced elasticity. The critical point is that chlorine degradation continues after you leave the pool. As long as the swimsuit is wet with pool water, the chlorine/hypochlorous acid continues reacting with the elastane. A swimsuit left damp in a swim bag for hours after a pool session suffers hours of additional polymer degradation compared with one rinsed immediately. The reaction rate slows as the chlorine outgasses and the concentration drops, but the process continues throughout the drying cycle. This is why the single most effective maintenance step is to rinse the swimsuit with clean tap water immediately after leaving the pool — even a 60-second rinse substantially dilutes the chlorine concentration and stops most of the ongoing degradation. Standard elastane (Lycra/Spandex) versus chlorine-resistant elastane: many competition and fitness swimsuits are made with chlorine-resistant fabrics where the elastane component is replaced with PBT (polybutylene terephthalate) polyester or polyamide elastomer rather than conventional polyurethane. These materials are significantly more resistant to chlorine attack because they lack the urethane linkages that hypochlorous acid preferentially cleaves. A standard lycra swimsuit loses approximately 40–50% of its elasticity after 150–200 hours of pool exposure; a chlorine-resistant PBT suit retains its stretch significantly longer. If you swim in chlorinated pools multiple times per week, chlorine-resistant fabric is worth the higher cost. Saltwater and UV damage work via different mechanisms. Saltwater (seawater) is primarily osmotic stress and UV photo-oxidation — salt increases the osmotic gradient across the fibre, drawing water in and out repeatedly as the suit wets and dries. This creates mechanical stress on the polymer structure. UV light (particularly UVA) photo-oxidises the polyurethane elastane via a radical chain mechanism, producing carbonyl chromophore groups that absorb visible light and cause yellowing. UV damage is cumulative and is worsened by leaving the swimsuit to dry in direct sunlight — a common and harmful habit. Sunscreen (particularly avobenzone-based chemical sunscreens) causes a specific type of staining on swimsuits. Avobenzone is a chelating molecule that forms a highly coloured complex with iron (Fe³⁺) ions — the same reaction that causes orange rust-like staining on white swimsuits. The iron source is usually tap water, metal pool equipment, or hard water deposits. Oxybenzone and other UV filters in chemical sunscreen also react with chlorine in pool water to form chlorinated aromatic compounds that deposit on the fabric and are difficult to remove. Rinse off sunscreen before entering a chlorinated pool when possible. Tumble dryers are the most damaging drying method for swimsuits. The combination of heat and mechanical tumbling targets both components: polyester and nylon shed structural stress from the spinning motion, but elastane is the primary victim. Above approximately 75–80°C — well within the range of a standard dryer — elastane polymer chains enter a thermoplastic flow regime and lose their elastic set (memory). One tumble dry cycle on medium or high heat can permanently reduce the suit's elasticity. Air dry only, and air dry flat rather than hanging, since the wet weight of a saturated swimsuit stretches the elastic panels when hung.