Air monitoring stations as far away as northern Wisconsin tipped off state regulators that something was amiss.

The readings on equipment at five locations revealed a sudden spike in mercury emissions, a hazardous air pollutant, on the evening of July 4 and part of the following day.

But where did it come from?

It turns out that mercury vapors were escaping up a chimney over a 19-hour period at a recycling facility, Veolia ES Technical Solutions, in Port Washington.

The incident is the first time that Department of Natural Resources officials say air monitoring equipment used to chart long-term trends in mercury in the atmosphere have detected a surge of mercury vapors before the problem was discovered by the plant operator.

Officials said that readings from ultra-sensitive monitors as close as Milwaukee and as distant as Rhinelander revealed mercury vapors at unusually high levels. Its detection in northern Wisconsin, more than 200 miles away, underscores the ability of mercury to travel long distances.

The concentrations being reported did not violate the state’s health-based standard and did not pose an immediate public health risk, but the results signified at the time that something was wrong.

The DNR says it is continuing to investigate the situation.

Andrew Stewart, field operations director for the DNR, said the mishap was a “serious event,” but citizens were never in harm’s way.

Katie Praedel, chief of air monitoring, said that the agency noticed data showing elevated emissions after employees returned to the office after the July 4th holiday.

Meteorological staff went to work studying wind patterns from that night. Other employees checked records for the small number of companies with air permits that handle mercury.

DNR staff concluded the likely source was Veolia, which operates household hazardous waste programs and incinerates products that contain mercury, such as batteries, electronics, lamp ballasts and dental amalgam, and contacted the company.

In a report to the DNR, the company said mercury vapors are normally sent through activated carbon filters, but a malfunction of equipment allowed vapors to go up a flue stack lacking such controls.

In a statement issued Thursday, the company said it "experienced a disruption" to its recycling process and immediately shut down its operations. Equipment is being modified, the company said.

Mercury emissions are regulated in Wisconsin and other states as a hazardous pollutant. In this case, Veolia released an estimated 1.54 pounds of elemental mercury, the company reported to the DNR.

By comparison, Milwaukee-based We Energies released about 100 pounds of mercury during 2016 from its coal-fired power plants, according to a company report.

The highest concentration from Veolia was measured at a monitor at the Horicon Marsh in Dodge County.

People exposed to elemental mercury at high concentrations can sustain severe lung damage, according to the federal Centers for Disease Control and Prevention, and can suffer neurological problems when exposed to low levels over time. The DNR says the levels released by the plant were below the allowable limit for 24-hour exposure.

Elemental mercury can also pose health problems as it falls on the landscape. Mercury can accumulate in the tissue of fish, exposing people to contamination.