Open fireplaces pose more risk of severe accidents inside the home than do woodburning stoves. Open fireplaces also produce a large amount of creosote residue. The amount of creosote produced depends not only on the type of flame but on how often the stove or fireplace is used. The most efficient wood-burning stoves produce less creosote than do many open fireplaces.
Although wood-burning stoves are more efficient than open fireplaces, they are also more dangerous. The smoke that wood-burning stoves release up the chimney is cooler than the smoke from an open flame. Thus it travels more slowly and deposits more creosote, a flammable substance that can clog a chimney -- or worse ignite inside it.
Which one of the following, if true, most seriously weakens the argument?