The United Nations says a treaty banning the world's most dangerous pesticides and industrial chemicals will take effect in May, now that the required number of countries has ratified it.

France this week became the 50th nation to ratify the 2001 Stockholm Convention on Persistent Organic Pollutants (POPs), and a U.N. statement Wednesday announced it will take effect May 17.

The accord will outlaw 12 potentially deadly and deforming chemicals which, experts say, travel through the environment and stay in the ground and water for years.

U.N. environmental chief Klaus Toepfer calls POPs the world's most dangerous pollutants. He says these highly-toxic chemicals cause cancer in humans and wildlife. He says they also damage the nervous, reproductive and immune systems, and lead to birth defects.