If the helmet is really stinky squirt the inside with dish soap, let it soak, then hose it down with a garden hose and let it dry in the sun.
__________________
The Lord is merciful and gracious, slow to anger and abounding in steadfast love. - Psalm 103:8
I am a cyclist. I am not the fastest or the fittest. But I will get to where I'm going with a smile on my face.