Deadly carbon monoxide gas leaking into a church sent six people to the hospital in Madison, WI, early Sunday afternoon.

People started feeling sick during Sunday morning services at Trinity United Methodist Church. An ambulance was called after a 2-year-old child became groggy and lethargic. Then other children began and adults reported feeling sick and dizzy.

After detecting high levels of carbon monoxide in the church with air quality monitors, fire officials evacuated everyone.

The source of the carbon monoxide is still under investigation, but authorities reportedly suspect the furnace system is faulty.

Carbon monoxide poisoning is called the silent killer because it is a colorless, odorless gas that can kill in minutes.

Every year, about 500 people die of carbon monoxide poisoning in the United States and as many as 20,000 visit an emergency room for exposure, according to the Centers for Disease Control and Prevention. Most problems come from poorly-maintained heating systems or gas stoves and gas-powered generators used for heat or power during storms.

Carbon monoxide is produced whenever any fuel such as gas, oil, kerosene, wood, or charcoal is burned.

According to the Environmental Protection Agency, symptoms of carbon monoxide poisoning include:

At moderate levels people can experience severe headaches, dizziness, mental confusion, nausea or fainting and death can occur if these levels persist for a long time.At low levels people can experience shortness of breath, mild nausea, and mild headaches and this may have longer term effects on your health.Since many of the symptoms are similar to those of the flu, food poisoning, or other illnesses, individuals might not think that CO poisoning could be the cause.People are advised to get fresh air and seek medical treatment if they are exposed to carbon monoxide.