When evaluating a 20 Bar (approx. 300 PSI) rubber air hose, you must prioritize the Working Pressure (WP) to Burst Pressure ratio, which should ideally be 3:1 or 4:1 to ensure safety. The material composition is critical; look for NBR (Nitrile Butadiene Rubber) or SBR (Styrene Butadiene Rubber) for the inner tube to ensure oil and mist resistance. The reinforcement layer should consist of high-tensile synthetic textile yarn or braided fiber to maintain structural integrity under pressure. Additionally, verify the temperature range, typically -30°C to +80°C, to ensure the hose remains flexible in cold environments and stable in high-heat industrial settings.