Amid mounting scientific evidence that no level of lead is safe for young children, a City Council committee Wednesday approved the biggest change - and, opponents argue, the costliest for both landlords and tenants - in at least 15 years in how Philadelphia deals with lead.

The bill would require the owners of most older rental units in the city to inspect and certify them as free of lead hazards every time a new tenant moved in if more than two years had passed since the last check.

The law already prohibits landlords from renting properties with lead hazards. But more than a dozen public-health advocates and physicians from several medical schools testified that without certification being required, many owners ignore the law. The city health department estimated the added cost of inspection and certification at less than $100 per unit every two years under an amended version of the bill that was unanimously approved late Wednesday by Council's Public Health and Human Services Committee. It is scheduled for introduction in Council on Thursday.

Far more landlords than advocates turned out for the hearing, and they predicted far more dire consequences - short-term rent increases of hundreds of dollars and permanent rises of perhaps $25, which they said would drive low-income renters to unsafe, uncertified housing.

"This is a feel-good bill" that will hurt low-income residents, said investor and landlord Jim Sims, who added that targeting most rental units was a waste since lead is a concern mainly for toddlers. About 1,000 city children a year, or 3 percent of those tested, have elevated levels.

Hanging over the proceedings were two opposing trends:

Levels of lead in children's blood nationwide have plummeted during the last three decades, due largely to a series of federal bans of lead in gasoline, food tins, and, in 1978, house paint. The main source of lead poisoning now is paint chips and dust ingested by children crawling in deteriorating houses built before that ban. More than 95 percent of houses in Philadelphia and more than 75 percent in Pennsylvania were built before it.

Over the same 30 years, the amount of lead that has been found to cause cognitive deficits in children has also plummeted. Two weeks ago, a federal advisory committee voted to essentially halve the level of concern set in 1991, to 5 micrograms per deciliter of blood from 10. By that standard, more than 9,000 children in New Jersey had elevated lead levels last year, and perhaps twice that number in Pennsylvania, including several thousand in Philadelphia.

Because it could not determine that any level was safe, however, the Advisory Committee on Childhood Lead Poisoning Prevention will also propose that government agencies focus less on blood measures and more on the environments - such as paint in old houses - that are known to expose children to lead, said member Deborah A. Cory-Slechta, a professor at the University of Rochester School of Medicine.

Rochester is one of about a dozen cities that have strengthened lead enforcement in recent years. Each is different; Rochester's own inspectors do the initial certification, which costs the city but not the landlords. Ugly consequences were also predicted before that law took effect in 2006, however, but a recent study found little effect on the rental market and a significant drop in lead levels.

Philadelphia's ordinance is modeled partly on a Washington law that took effect in 2009 and also is credited with reducing lead levels in children. That law requires landlords to hire certified inspectors but, in the first phase, only for units whose tenants-to-be include a pregnant women or a child younger than 6.

A quick survey found that the process "has not proven to be difficult, although it is expensive ($300 or so) and can delay the new resident's occupancy by several days" and "is a significant administrative burden," W. Shaun Pharr, a senior vice president of Washington's Apartment and Office Building Association, said in an e-mail Wednesday.

Philadelphia's landlords are likely to press Council to consider a compromise such as the one that Washington accepted, limiting the law's reach to units with young children or pregnant women, at least initially.

Nan Feyler, chief of staff for the city Department of Public Health, said she strongly supported keeping the bill as is because, among other things, a family that is childless might not stay that way. She said an amendment that made it into the bill late Wednesday, allowing landlords to have their own employees trained and certified to conduct inspections, was a "big compromise" that would cut costs.

Current city law forbids landlords from renting units with unsafe levels of lead. But the city finds out about violations mainly after it receives a report of a child with elevated lead levels and goes out to inspect, meaning that high-risk units without children may not get discovered. It can order remediation, and sometimes provides funding from the federal government to pay for it.

"All the [proposed] law is requiring is what landlords are already required to do under the housing code," said Feyler, adding that the city's intention was "to not put an additional burden on landlords than already exists."

She said Mayor Nutter supported the bill.

The measure has changed several times since it was proposed two years ago by Councilwoman Blondell Reynolds Brown, who chaired Wednesday's hearing.

The version that will get a first reading in Council on Thursday - and which Reynolds Brown hopes to have passed next week - applies to all rental properties built before March 1978, excluding student housing and units owned by the Philadelphia Housing Authority, which Feyler said had its own requirements.

Under the bill, landlords would have to certify to the prospective tenant and the city that the unit was safe based on visual inspection and a specific "dust wipe," but, Feyler said, the unit would not need to meet stricter standards for lead abatement that can often cost thousands of dollars.

The unit would have to be recertified every time a new tenant moved in unless it had been cleared within the previous two years. The bill, which would take effect in one year, does not cover owner-occupied units.

That was one of several provisions that angered landlords. It would be like "putting a Band-Aid on a bullet wound," said Norman Bey, one of several representatives testifying for the Homeowners Association of Philadelphia.