So just how gross do festival wristbands get?

And many people decide to leave them on for a while as a part souvenir/part badge of honour ("Oh yeah, I’ve been at Glasto three times. No big deal.").
The more dedicated festival-goers tend to accumulate a decent amount over time, charting their summer from Body and Soul to Electric Picnic, getting grubbier and grubbier as the months and even years pass.