The controversy over fluoride has long centered on health, but this week in Salem, the tough questions will be about its cost.

Senate Bill 99 would require towns of 10,000 or more to add the cavity-fighting chemical to their drinking water. Most of those towns, through the League of Oregon Cities, are bristling at the mandated expense.

“It’s a local issue, and it’s been voted on,” says Susan Schneider, a lobbyist for the city of Portland. “Fluoridation is costly.”

The main expense is not fluoride: As a waste product of the fertilizer industry, people are practically giving it away. The real cost comes as the corrosive chemical dissolves the lead in old plumbing. In order to avoid busting the Clean Water Act’s lead limits, Portland would have to buy $2.5 million in new equipment. Mark Knudson, the water bureau’s operations manager, wrote in a recent email to Schneider that SB99 “would have a MAJOR impact.” (The bill is scheduled for a committee hearing April 11.)

Fluoridation advocates argue that by reducing dental costs to the Oregon Health Plan, SB99 would more than recoup its costs over time. As of last week, no one had estimated what those costs would be statewide. But when a draft of the bill circulated last year, Knudson wrote that numbers provided by the health division and Oregon Dental Association underestimated start-up costs “by a factor of perhaps 10 or more.”