CHICAGO (CBS) -- The National Weather Service has issued an excessive heat warning in effect for Chicago and Cook County through Monday at 7:00 p.m.

The collar counties and other parts of Illinois are under a heat advisory until midnight.

Heat is the number one weather-related killer. There will be a prolonged period of dangerously hot temps, with little relief at night through Monday.

The heat warning was issued because there will be three consecutive days with a 100-104 degree index.

In the city, the urban heat island effect and a lack of air conditioning can put people at risk.

During a heat wave, the elderly and poor are at higher risk.

Stay hydrated and check on neighbors. Heat has a cumulative effect on the body.

Anyone without air conditioning should visit a cooling center, store or library to spend some time cooling down.