Synthetic fibres are petroleum-derived polymers: polyester, nylon, acrylic, polypropylene, elastane. Advantages: durability, wrinkle resistance, packability, low cost. Disadvantages: low breathability, microplastic pollution, non-biodegradable. Sustainability score 3 (worst). Recycled variants (recycled-poly, Econyl) reduce virgin plastic use while retaining performance properties.