SPF (sun protection factor), on the other hand, refers to the level of protection provided by sunscreen against only UVB rays. The higher the UPF or SPF number, the greater the protection.

UPF clothing is a growing billion-dollar market. Data from Shanghai-based China Insights Consultancy found that China's sun protection apparel market will grow at a compound annual growth rate of 9.4 per cent from 2021 to 2026, with the market size reaching 95.8 billion yuan ($$17.8 billion) in 2026.

According to Dr Michelle Wong, a doctor at IDS Clinic, a fabric with a UPF 50 rating allows only 1/50th of the UV radiation to pass through it - which makes for "excellent sun protection".

While sunscreen efficacy diminishes with time, UPF clothing is highly effective in providing long-lasting UV protection, she says. However, factors such as wear and tear, stretching and washing can reduce its effectiveness over time. UPF clothing is specially designed with tightly woven fabrics, special dyes or chemical treatments to provide enhanced UV protection compared with regular clothing.

The average UPF level of a regular shirt is relatively low, typically ranging from UPF 5 to UPF 15. A typical white cotton T-shirt offers about a UPF 5 rating, which means that one-fifth of UV radiation passes through it. Fabrics rated below UPF 15 are not considered UV-protective, she adds.

And not all colours are equal in sun protection. While black or dark colours might seem unappealing on a hot day, they offer better protection than lighter shades, she says. "Dark or bright colours keep UV rays from reaching your skin by absorbing them rather than allowing them to penetrate."

If UPF clothing is out of reach, Dr Wong suggests looking for clothing with a tight fabric structure, or densely woven fabrics such as denim, bamboo, wool and certain synthetics - which may be uncomfortable, but are "more protective than sheer, thin or loosely woven cloth".

"In hot climates like in Singapore, opt for UPF clothing made from lightweight, breathable fabrics with moisture-wicking properties. Loose-fitting styles and lighter colours can also help in staying cool while maintaining UV protection," she says. "Additionally, consider wearing UPF accessories like hats and sunglasses for added protection."

First published on The Straits Times