The Esmond family is still being treated after being exposed to methyl bromide while on vacation in St. John

Sean and Ryan Esmond remain in critical condition six weeks after being exposed to a banned and highly toxic pesticide while on vacation with their family at a resort in the U.S. Virgin Islands.

The teen brothers are currently in a Philadelphia hospital, while their parents, Steve Esmond and Theresa Devine, continue to undergo rehabilitation therapy, according to a statement released by the family’s lawyer.

The family fell ill at the Sirenusa resort in St. John on March 20. Paramedics found Esmond unconscious, and his wife and sons suffering seizures.

Preliminary testing of the family’s unit found the presence of methyl bromide, an acutely toxic and odorless pesticide that is mostly banned in the U.S.

The condominium beneath the family had been sprayed with the pesticide on March 18, a resort official told The News Journal in Wilmington, Delaware.

“Human exposure to high concentrations of methyl bromide can result in central nervous system and respiratory system failure,” the EPA explains on its website. The agency banned residential use of the chemical in the U.S. in 1984.

The teenagers’ prognosis depends on the duration of exposure and how much they breathed in, Dr. Reynold Panettieri, deputy director for the Center of Excellence in Environmental Toxicology at the University of Pennsylvania’s medical school told the Associated Press.

“The potential for meaningful recovery is still there,” says Panettieri, who is not involved in the teens’ treatment. “As you get farther and farther out, the potential for meaningful survival and living independently is going to become less and less likely.”