BOSTON (AP) _ More than one-fourth of the children poisoned by lead in Boston during the past five years came from 22 ''hot spots,'' areas averaging four to five blocks in size, according to a new study.

Soil in one yard in the Dorchester section had 26,000 parts of lead per million, more than 50 times the 500 parts per million that the federal Centers for Disease Control considers hazardous to children.

The hot spots generally are ''neighborhoods of closely spaced wooden houses in poorer sections of the city,'' said Richard Rabin, a researcher for the city lead program.

He said many yards don't have grass, making soil contaminated over the years by flaking lead house paint accessible to children at play.

Lead is especially poisonous to children younger than 6 because their developing nervous systems are more vulnerable than those of adults. Studies indicate that children may suffer permanent brain damage from chronic exposure to even low levels of lead. The damage may appear later as learning disabilities, emotional disturbances or attention disorders.

Lead contamination is one of the city's most serious toxic hazards, said Ronald R. Jones, head of the Childhood Lead Poisoning Prevention Program at Boston City Hospital.

''There are strong arguments for these areas being considered emergency areas under the federal Superfund law,'' he told The Boston Globe. That law has been used almost entirely to find and clean toxic waste dumps.

Jones said lead concentrations in the 22 areas averaged 2,000 parts per million. The study found that in the last five years, 757 children who were poisoned by lead, or 27 percent of the total cases in Boston, came from those areas.

The average level of lead in Boston soil is 700 parts per million, said Alan Schepps of the Suffolk County Cooperative Extension Service, which tests soil for home-owners.

Michael Deland, regional administrator of the Environmental Protection Agency, agreed that lead was a serious contaminant but said the EPA has not determined the size of the problem in Boston or ''how serious it is in relation to other health problems we deal with daily.''