To confirm your head circumference, you can use a flexible tape measure to measure the widest part of your head:
Place the tape measure in the middle of your forehead, just above your ears.
Wrap the tape around the widest part of your head, which is usually around the back of your head.
Pull the tape snugly but not too tightly.
Remeasure 2-3 times and record the largest number.
If you don't have a tape measure, you can use a piece of string and measure its circumference with a ruler.
If your measurement falls between two sizes, it's best to round up.

CPSC certified means thata product has been tested and certified by the U.S. Consumer Product Safety Commission (CPSC) to meet consumer product safety requirements.The CPSC is a federal agency that protects the public from injury or death caused by consumer products that pose a risk of fire, electrical, chemical, or mechanical hazard.

The effective period of a bike helmetdepends on how often and how you use it, but experts recommend replacing it every 3–5 years:

• Frequency of use:If you ride your bike daily, it will be more exposed to sun and general wear and tear. 
• Crashes:Even a lighter crash can affect the helmet's EPS padding, straps, and fit systems. 
• Sun exposure:UV rays can affect the helmet's shell, making it brittle. 
• Material deterioration:The material in a helmet will begin to deteriorate over time, making it less effective. 

You should also replace your helmet if it shows obvious signs of wear, such as dents, scratches, or cracks.These flaws can compromise the helmet's structural integrity and put you at a higher risk of head injuries.You can regularly clean and care for your helmet to help you identify damage or deterioration.If you're in doubt, it's better to replace it than not.

To care for your bicycle helmet, you can:

• Store it properly:Keep your helmet in a cool, dry place out of direct sunlight and away from extreme heat. Avoid storing it in your car trunk or attic. 
• Clean it regularly:Wash your helmet with mild soap and water, a soft cloth or sponge, and warm water. You can also use a warm, wet towel to loosen dirt. If your helmet has removable pads, you can wash them in a washing machine on a gentle cycle. 
• Avoid chemicals:Don't expose your helmet to harsh chemicals like bleach, paint, or insect repellants. These chemicals can weaken the helmet and make it less effective. 
• Check for damage:Regularly inspect your helmet for dents, cracks, tears, or rips. 
• Replace it when needed:You should replace your helmet if it's more than three years old, or if it's been exposed to a lot of wear and tear or extreme environmental conditions. Newer helmets may also have better technology and fit more comfortably.