How could a thin ozone layer harm humans?

The ozone layer prevents many harmful rays from reaching earth. Without this, the UV rays could destroy plants and animals, cause cataracts, and even cause skin cancer.

When did most countries stop producing CFCs?

The Montreal Protocol is an international agreement which was drawn up in September 1987. It originally aimed to half the use of CFCs by 1999. However, reviews of the protocol held in 1990 in London and 1992 in Copenhagen imposed more stringent controls, so that all production of CFCs, CCl4 and halons should cease by the year 2000. Many countries have even agreed to stop using CFCs before this deadline. Production of other ozone depleting gases is to stop in the early years of the 21st century.