It was the chemical contamination of drinking water, not carbon monoxide poisoning, that sickened more than 20 office workers at the Johns Hopkins at Keswick complex earlier this week.

Johns Hopkins and Baltimore City health officials are blaming a faulty water heater, which allowed nitrites and nitrates to mix with the drinking water, for causing dizziness, headaches and difficulties in breathing reported by the employees. The chemicals are used to prevent rust in boiler tubes.

Initially, CO was thought by some employees to be the cause of the sickness, which resulted in the two buildings at Keswick Rd. and 40th St. to be evacuated on Tuesday.

Here is more information from a Hopkins release to employees:

“Diligent testing identified a hot water heater in the south building of the Keswick complex. The heater harbored a source of nitrates and nitrites that made their way into hot water service in the north half of Keswick’s south building.

“This discovery fits with the facts as we have come to understand them over the past two days.

“First of all, nitrates and nitrites can cause the types of symptoms that employees reported. Second, the north side of the south building, where running water with those chemicals was found, is served by that hot water heater.

“Third, that is also the side of the building where sickened employees have reported coming into contact with water in various ways: eating food cooked with hot water, drinking beverages made with hot water, or using restrooms.

“Now that the hot water heater has been identified as the problem, it has been cut out of the system.

“While we continue our investigation, all restrooms, sinks, and other water sources served by that hot water heater will be closed. Once we have thoroughly tested and cleared all such sources, they will reopen.

“Public health authorities working collaboratively with us will continue their work to further characterize this episode and draw appropriate lessons from it.

“We will investigate what caused this water heater to become contaminated, and put measures in place to prevent a repetition. The heater itself will be replaced.”