The warning came a day after firefighters extinguished a large fire at the International Terminals Co. southeast of Houston that began Sunday and sent a huge plume of black smoke into the air that could be seen for miles.
Here is a look at the situation:
Q: What is benzene?
A: Benzene is a colorless chemical found naturally in crude oil, gasoline and cigarette smoke. But it also is used widely in industry to make plastics, resins, lubricants, rubbers, dyes, pesticides and other products. Most of the storage tanks that burned at ITC contained components of gasoline, including benzene, company officials have said.
Because benzene is very flammable, firefighters have continued to put thick layers of suppressant foam on the areas that burned to prevent a flare-up. Authorities said Thursday that it appeared the foam was separating at times and allowing benzene vapors to escape.
ITC officials also said Thursday that crews were working to empty a storage tank they believe emitted benzene vapors, but the site was still warm from the fire and the company was using a drone to try to determine how hot the temperatures were on the ground.
High benzene levels were detected by monitors as far as six miles from the site, in a Deer Park neighborhood, according to Elena Craft, a senior health scientist at the Environmental Defense Fund.
Q: Why is benzene a health concern?
A: Breathing high levels of benzene can cause drowsiness, dizziness, rapid heart rate, headaches, tremors, confusion, and unconsciousness, according to the Centers for Disease Control and Prevention. Long-term exposure affects the bone marrow and can cause anemia and leukemia.
When benzene gets into the air, it breaks down within a few days, though it can attach to rain droplets or snow and be carried back to the ground. It also can contaminate the air by evaporating from water and soil.
Some residents in Deer Park said they decided to leave rather than shelter in place.
Q: Have there been other problems with high benzene levels?
A: High levels of benzene were found in the Houston neighborhood of Manchester in 2017 after Hurricane Harvey. The Houston Health Department said that high levels were detected close to a Valero energy refinery at levels above which workers are advised to wear breathing equipment.
Last month, Drummond Company agreed to pay a $775,000 fine because of benzene air violations at its ABC Coke plant in Tarrant, Alabama, just north of Birmingham. A 2011 inspection found that wastewater containing benzene was escaping to the air through improperly sealed pipes and equipment.
Q: Are there any other concerns?
A: Environmental Protection Agency spokesman Adam Adams said air monitoring has not detected any "continuous" hazardous conditions. He said the agency is using a specially equipped bus and a small airplane to conduct air monitoring throughout Deer Park.
Craft said the Environmental Defense Fund and researchers from Texas A&M University planned to collect water samples Friday from Galveston Bay to measure what pollutants, including benzene and toluene, might be flowing from the shipping channel where ITC is located. She said they will have real-time results for most pollutants.
Craft said Texas A&M will collect samples to test for perfluoroalkyl and polyfluoralkyl compounds, or PFAS, which are found in firefighting foam, but those results would not be available for a few weeks.
There is growing concern that long-term exposure to PFAS can be harmful.
Associated Press reporter Juan A. Lozano contributed from Houston.
Follow Webber at https://twitter.com/twebber02