Breathability measures a fabric's ability to allow moisture vapour and air to pass through it. Quantified by air permeability (cm³/cm²/s) per ISO 9237 or Moisture Vapour Transmission Rate (MVTR). High breathability (0.8-1.0 normalised) reduces heat build-up and sweat accumulation. Natural cellulosic fibres (linen 0.95, hemp 0.90) score highest. Dense synthetics (polyester 0.30) score lowest. Breathability amplifies in hot-humid climates where fabric-skin contact traps heat.