Old smell coming out of backpack?

I bought a nice, barely worn backpack through the classifieds.

The backpack smells a bit musty. I assume it was sitting in the basement.

I've already let the backpack air out for two weeks indoors and two days outdoors at sub-zero temperatures.

Now I've brought it back inside and spray it with lemon water (water and lemon oil) several times a day.

But the backpack still smells. It can't be washed.

Does anyone have any tips on how I can get rid of the smell?

I cannot use perfume or similar substances because I have asthma.

Best regards and thank you 😊