The unknown illness has caused at least five deaths according to health authorities in Luanda, the capital, and some 64 percent of those affected are children, WHO said in a statement.
After conducting blood tests in laboratories in Germany, WHO found "very high levels" of bromide in every five out of six blood samples tested.
"Additional samples are currently on the way to a laboratory in Britain for testing for bromides," WHO added.
On Monday, more than 390 cases were identified and treated at the municipal hospital in a suburb of the Angolan capital.
On Wednesday, WHO deployed more technical and operational support to aid Angolan authorities.
All the victims have complained of dizziness, sleepiness, and difficulty in walking and speaking.
According to the WHO website, methyl bromide is primarily used as a fumigant in ships' holds, grain silos and other large enclosed areas.
The health agency has not indicated what could be the origin of the illness, but the local press pointed out the amount of pollution in that particular suburb as a possible culprit.
In October, inhabitants said that a heavy green cloud came from a small petrochemical factory which has since been closed, according to the financial daily A Capital.