A mystery disease that has hit nearly 400 Angolans is associated with very high levels of the chemical bromide, UN health experts say.
More than half the patients are under 15. At least four people have died.
The first case of the disease was reported in October in the capital, Luanda. Symptoms include fever and extreme drowsiness.
The World Health Organization is sending extra staff to Angola to treat people and investigate the outbreak.
They include doctors and experts in food safety, logistics, and laboratory work.
The WHO said initial tests done in Germany found "extremely high" levels of bromide in five of six blood samples taken. The results are due to be checked at a laboratory in the UK.
WHO officials said last week they were mystified by the disease.
If bromide is indeed the cause of the outbreak, it is not clear how patients were exposed to it.
Bromide is sometimes found naturally - such as in sea water - but in other forms can be toxic or even fatal.