The severity of an Iowa City landfill fire is an unintended consequence of a state rule that encouraged the site to use an extra layer of tire chips to protect its lining from winter frost, a city official and a consultant said Wednesday.

But a Department of Natural Resources regulator said that the landfill had other options to comply with the rule and that operators can limit fires by better managing risk when they use shredded bits of tires for drainage and frost protection.

The fire at the Iowa City Landfill burned Wednesday for a fifth day in an unused part of 7.5 acres of a landfill cell. Smoldering tire chips — not trash — are producing hazy, black smoke. City officials said the fire, which they are letting burn out on its own, could last several more days. Public health officials have advised residents to minimize exposure to the smoke.

City officials said the fire started Saturday when a load containing flammable material — such as used charcoal or ashes — was dumped. The flames quickly spread upon reaching the lining of tire chips, which were exposed in about half of the cell that hadn't been covered with trash.

With encouragement from regulators, landfills across the country have used tire chips since the 1990s as a way to collect and drain liquids that flow through trash heaps. Proponents say the chips are a good way to recycle millions of tires and are more effective at draining than sand....