Google has shut down two buildings close to its headquarters in Mountain View, Calif., ordering all staff to evacuate until further notice as the air in those buildings has become too toxic and dangerous to work in.

“The health and safety of our employees is Google’s number one priority, and we take several proactive measures to ensure the healthiest indoor air environments possible,” the company said in a statement to CBS.

The cause of the dangerous fumes is a toxic solvent called trichloroethylene, or TCE, which is typically used for industrial purposes like degreasing metal parts, but is highly toxic to the human central nervous system and is highly associated with cancers in the liver and kidneys, as well as Parkinson’s disease.

And while Silicon Valley has many great traditions, dumping chemicals – particularly toxic ones like TCE, ethylbenzene and vinyl chloride – was unfortunately a very popular one three decades ago. Back when Intel, Raytheon and Fairchild owned Google’s current property, those companies dumped thousands of gallons of TCE into the ground, which seeped into the soil and polluted the water – hence the toxic air eruptions the area still experiences today.

Since the early 1980s, when cleanup efforts began, the EPA says more than 2,000 pounds of volatile organic compounds and 74 million gallons of infected groundwater have been removed and treated. Treatment of the soil continues to this day, with new systems being installed for extracting and treating infected water and air, as well as “slurry walls” that plow deep into the earth, vapor barriers and vapor pumps that can blow toxic fumes away from Silicon Valley’s residential areas.

Google knew about the toxicity issue when it moved into the area in 2004; its two affected buildings, QD6 and QD7, were given state-of-the-art air filters and frequently tested for air quality to ensure safety from pollution.

The EPA says a “normal” screening level for commercial buildings is 5 micrograms per cubic meter: And while many of Google’s air sample stations in QD6 and QD7 detected TCE levels well below that, roughly a dozen stations gave off readings between 5 and 30 micrograms per cubic meter. One particular station measured 120 micrograms per cubic meter.

The EPA says Google employees have likely been exposed to the toxic air for “months,” but the agency says it “takes decades of exposure to cause problems.” In a recent air sampling from the EPA, “heightened levels of TCE” were discovered, but the causes of the leaks were reportedly fixed, and Google did not have to evacuate its buildings.

That said, a 2012 study found a spike in TCE-related cancers in the area surrounding Google’s HQ in Mountain View; in fact, Silicon Valley ranked No. 8 on Forbes’ most recent annual list of America’s Dirtiest Cities. Google is currently working on building a new headquarters in London, which is expected to be finished in 2016.

We have reached out to Google to comment on what is being done to combat the pollution to reopen those two buildings, and we will update this story as soon as we learn more.