Customers at the Walmart on Sunset Boulevard in Lexington were reporting irritated eyes Sunday.

It was a sign they were facing a potentially deadly situation.

Firefighters responded to check on air quality inside the store and confirmed elevated levels of carbon monoxide.

Customers and employees were evacuated while the Walmart was ventilated.

Investigators say the CO fumes were coming from a faulty heating unit.

Exposure to the toxic gas can cause headaches, dizziness, weakness, nausea, vomiting, confusion...even death.

The Centers for Disease Control and Prevention says more than 20,000 Americans go to the emergency room annually due to unintentional CO poisoning, more than 4,000 are hospitalized, more than 400 die.

Thus, the advisories to properly ventilate gas or other fuel burning appliances, and to use a carbon monoxide detector (pictured) in your home or business because the gas is odorless and colorless.