The Valley's excessive heat advisory has been extended through Sunday, and Phoenix Firefighters are urging you to take steps to guard against the searing temperatures.

Captain Jonathan Jacobs says that means planning the day before if you know you'll be spending time outdoors. And, he says, there's no substitute for drinking lots of water, whether you plan to go hiking or just work in the yard.

"Pre-hydrate, and then hydrate while you're doing your activity," Jacobs said. "Depending on your level of being in this heat, and your tolerance to that, is all going to depend on how you hydrate your body."

Jacobs says dressing right in the extreme heat means hats, cotton attire and long sleeves. And he suggests rugged footwear, as opposed to sandals, for hiking and says you also need to pack several bottles of water.

For the third straight day environmental regulators are issuing a health advisory for high ozone in the Phoenix area, as searing heat and easterly winds are expected to cause a buildup of the pollutant. ADEQ says Tuesday's ozone levels were some of the highest of the year and they should be very high again Thursday.

The invisible gas is created when pollutants in the air, like those created by vehicle engines, are heated by the sun. Experts say ozone can irritate the lungs and aggravate existing health conditions such as asthma.