The ozone haze that has spread over Central Florida the past couple of days has triggered warnings that even healthy adults and children should avoid prolonged exercise outdoors.
People with breathing problems, such as asthma, are particularly vulnerable.
Ozone, a lung-burning pollutant, peaked at one of the highest levels ever recorded in Orange County on Tuesday and could remain elevated if sky-cleansing rains don't arrive after nearly a week's absence.
"Everything is stagnant, so we sit here and cook," said Merle Kruger, a senior engineer for the county's Environmental Protection Division.
The past week has brought less than a quarter-inch of rain to Orlando, though chances of wet weather will increase gradually to 50 percent Friday and 60 percent Saturday.
"We just have to hope for rain," said Hamp Pridgen, a program manager with Orange County's Environmental Protection Division.
For much of the nation, elevated levels of ozone, a corrosive and unstable form of oxygen, occur during warmer, sunnier months of summer, when auto and industrial air pollution react with sunlight.
Orange County's high reading -- also the state's highest -- came Tuesday afternoon, when ozone reached an hourly average of 120 parts per billion.
That was the highest since 1998, when wildfires swept the state and pumped volumes of ozone into the air for an hourly average of 133 parts per billion.
Reliable measurements of ozone in Orange County began in the early 1980s.
Other than during the outbreak of wildfires, the high mark was set in 1990 at 122 parts per billion.
Environmental officials say that improvements in pollution controls have steadily reduced ozone pollution.
The Environmental Protection Agency likens the potential effects of ozone to sunburn for lungs, resulting in coughing and difficulty breathing.