Governments have agreed to phase out the use of the toxic chemical HBCD, and restrict trade in four other dangerous substances, the head of the UN's anti-pollution division said Friday.

"Adding these chemicals to the list is a good thing, because they are known to be quite bad chemicals," Jim Willis, executive secretary of the Basel, Rotterdam and Stockholm Conventions, told reporters as a two-week international conference wrapped up in Geneva.

The conference agreed to ban the production and use of HBCD from next year, albeit with a five-year grace period for its use as a flame-retardant in polystyrene building insulation.

HBCD, or hexabromocyclododecane, is also used in interior textile fittings for vehicles as well as packaging materials. It is considered a "persistent organic pollutant" -- chemicals which linger in the environment, enter the food chain and thereby pose risks to human health and nature.

Health campaigners say that among its ills is that it undermines the ability of children to learn and grow because it can harm thyroid function and brain development.

Such chemicals are overseen by the Stockholm Convention, finalised in the Swedish capital in 2001 and which to date has drawn in 179 nations.

A separate accord, the 1998 Rotterdam Convention, restricts trade in chemicals by obliging exporters to ensure that destination countries have been fully informed about the risks involved and have given an explicit green light for imports.

A total of 152 nations have signed up to that accord, and the parties agreed to add four chemicals to its list: the insecticide azinphos-methyl; perfluorooctanesulfonates, which can be used as water repellents; and two forms of flame-retardant, pentabromodiphenyl ether and octabromodiphenyl ether.

Delegates failed, however, to slap similar trade restrictions on the pesticide paraquat, in the face of resistance piloted by India.

In addition, they were unable to reach a consensus on adding chrysotile asbestos -- which health experts say causes cancer -- to the list.

Past efforts to do so were long stymied by Canada, a major producer until the government withdrew support to the industry last year.

With Ottawa taking a back seat, the baton was picked up by Zimbabwe and Russia, the globe's top asbestos producer.

Unlike the 180-nation Basel Convention of 1989, which governs exports of toxic waste notably from rich to poor countries, the Stockholm and Rotterdam Conventions lack compliance mechanisms and have to rely on countries honouring their pledges.