CO2 makes carbon monoxide more toxic.

Eighty percent of the people who die in fires die from inhalation of smoke and toxic gases, not from flames, notes Barbara C. Levin, head of fire toxicology at the National Bureau of Standards' (NBS) Center For Fire Research in Gaithersburg, Md. NBS has launched a program to identify the most toxic combustion products and the possible synergistic effects that the hundreds of gases created in a fire might pose when allowed to interact. The goal is to build a computer model that can predict the probability of toxic-gas production and the risks those gases would pose to humans, in environments of specific room dimensions, furnishings and building materials.

Many of the toxicity data for this model are coming from new tests. And Levin reports that some of these tests have already begun yielding surprising data. For example, carbon dioxide (CO.sub.2) is normally considered nontoxic, she says. And though CO.sub.2 levels in a fire usually do not exceed 21 percent, she says, experimental rats could survive up to 40 percent CO.sub.2 in air before getting sick. Yet when CO.sub.2 and carbon monixide (CO) were delivered together for 30 minutes--a situation representative of fire conditions--concentrations of CO.sub.2 as low as 5 percent in air were enough to double the toxicity of the CO, which became lethal at just 2,500 parts per million in fair. Levin says the CO.sub.2 appears to exert its damage by increasing the rats' uptake of CO and by producing an increased and prolonged respiratory acidosis (toxic acidification caused by a buildup of carbon dioxide in blood).