Suffolk and Nassau each have more than 400,000 homes built before 1978, the U.S. Census Bureau estimates.

That’s the year lead paint was banned for use in residential homes because of its toxic and sometimes fatal effects, especially for children.

Starting April 22, a new federal rule kicks in for contractors, painters and other specialty trades when they renovate pre-1978 homes, schools and childcare facilities. Under the U.S. Environmental Protection Agency rule, contractors and others who do work in such buildings must be certified on lead-safe practices, from controlling dust in the renovation area to cleaning up thoroughly.

The rule was approved two years ago, and according to the EPA, about 50,000 renovators have been trained.

But the National Center for Healthy Housing, a Maryland-based advocacy group for safe and green housing, has voiced concerns that not enough contractors and homeowners know about the new rule. Contractors must be certified by an EPA or state-approved trainer; the nonprofit is one of them.

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Census figures show Nassau has about 79,000 renters in pre-1978 buildings and about 337,000 people in owner-occupied, pre-1978 homes. In Suffolk, it’s about 73,000 renters and 249,000 in homes, the Census said.

Lead can damage the nervous system, physical development and learning in children. Adults may get high blood pressure, muscle and joint pain, reproductive problems and other health risks.

For details, go to nchh.org or epa.gov/opptintr/lead/pubs/renovation.htm, where lists of certified contractors can be found.