I had estimated upwards of 90% given that the article is targeted at English-speaking adults, but https://onlinelibrary.wiley.com/doi/full/10.1111/iej.13822 suggests that I may have overestimated—across Europe only 60% of people had had a root canal, with less variation between countries than I had expected.

So maybe only 75% of the people reading the article have gutta percha in their mouths.