The temperature of a cool iron setting is often around 275 degrees Fahrenheit, while the highest setting is around 445 F. To prevent fabric damage, synthetic or delicate textiles like nylon and acrylic should be ironed on a low setting.
Acetate and silk are other materials that should be ironed on a low setting. Find the temperature knob on the front of the iron and turn it to the low setting. For the coolest temperatures, put the knob to 1 or 2 if it has a number scale to indicate temperature settings. Some irons additionally specify which fabrics are appropriate for each setting.