Size (H)5.5, (W)1, (D)5.8cm. White. Detects poisonous Carbon Monoxide (CO) gas. Sensitised spot changes from yellow to dark grey if CO gas is present. Position near gas, oil or solid fuel fired appliances or fuel driven engines. Safe, effective and affordable way of protecting your health. Use in the home, office, hotel rooms, caravans, boats and more.