Hundreds of thousands of tractor-trailer and bus drivers in the United States carry commercial driver’s licenses despite also qualifying for full federal disability payments, and some of those drivers have suffered seizures, heart attacks or unconscious spells, according to a new study.

According to the Fort Worth Star-Telegram, the 30-page GAO study, obtained by the AP in advance of its release this week, said 563,000 commercial drivers were determined by the Veterans Affairs Department, Labor Department or Social Security Administration to be eligible for full disability benefits because of health problems.

Truckers’ health problems, such as diabetic seizures and sleep disorders, have been blamed in numerous fatal truck wrecks across the United States. Of Texas’ 819,300 people with commercial driver’s licenses, 22,600 receive full federal disability payments. 16,100 of those 22,600 got their commercial driver’s license after they were certified as totally disabled. Most disturbing is that Texas allows drivers to self-certify that they meet medical requirements for a commercial driver’s license.