When people think of threats to the health of children, the usual things that come to mind are infectious disease, accidents or abuse.

But federal officials this week unveiled a billion-dollar, five-year plan to attack what they see as a major public health threat - lead poisoning in the blood system of youngsters.The figures are shocking. An estimated 3 million to 4 million American children - one in every six - absorb enough lead before they are six years old to cause irreversible brain damage and serious injury to the nervous system.

The poisoning comes from lead-based paint in older homes and public buildings, from lead-contaminated soil and dust, from drinking water carried in lead pipes, and from manufactured products containing lead in some form. The lead in paint is the most widespread source of contamination.

To those children who have gotten lead in the blood, the consequences are appalling. Young people with even moderate lead poisoning are seven times more likely to drop out of school and six times more likely to suffer reading disabilities and are more inclined to become addicted to drugs.

The level of lead considered hazardous has been shrinking constantly. Scientists once considered 40 micrograms of lead per deciliter of blood as dangerous. Twenty-five micrograms is now regarded as a hazard, and the danger level soon may be set at 10 micrograms.

The federal plan would clamp down on the use of lead in certain products, would further regulate lead in air and water supplies, would encourage more screening of children under age six, and would seek to remove lead-based paint from homes.

If all of this seems like an enormous undertaking, it is. For example, an estimated 57 million residences - 74 percent of all privately-owned homes - have some lead paint on their walls. Removing or encasing the lead would take $5,550 to $11,900 per home.

The cost of doing this obviously would run into billions of dollars, even if only the most contaminated homes in which young children live were to be dealt with.

Most of the money for rehabilitating older homes and screening children would have to come from local governments. Federal agencies, while focusing on the problem, do not have the massive resources needed just to begin. For that matter, neither do state and local entities.

But some combined federal-local effort will have to be made to address the problem. The cost of not doing anything would, in the long run, be even more expensive - both in ruined and wasted lives and the tax dollars needed to deal with the human results of lead poisoning.