There are no unusual health risks on Redang. Wear sunscreen and heed the warning flags on the beach, although they're more or less permanently set on green. Asian women travelling solo or in pairs may at times find themselves getting unwanted attention from local men working at resorts but it is relatively harmless and probably has its roots in cultural perceptions of gender roles.
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Wearing bikinis and skimpy beachwear is acceptable and going topless is not. However, due to the culture it is preferably if you wear your swimming clothes decently.