The Extreme Heat Warning over Maricopa County runs through Thursday, July 9, with daytime highs forecast between 112 and 116 degrees Fahrenheit [1]. That is the number on broadcast weather. The number that determines whether a Phoenix resident survives this week is different: the overnight low.

On Tuesday and Wednesday nights, Phoenix will record overnight lows between 86 and 91 degrees Fahrenheit [2]. For specific categories of people, that is not a sleeping temperature. It is a medical event that begins in the dark, compounds through the night, and announces itself at six in the morning — long before the thermometer climbs back to triple digits.

As this paper reported Monday, the heat dome moved west from the Northeast and the paper named Phoenix, Denver, and Las Vegas as the next care nodes. That forecast is now geography. The dome is in Phoenix, and the care burden has followed it.

The mechanism

The body's primary heat-shedding mechanism during sleep is vasodilation: widening of blood vessels near the skin allows the body to radiate heat into ambient air. For that mechanism to work, ambient air must be meaningfully cooler than core body temperature — around 98.6°F at rest, declining slightly during the sleep cycle. When the overnight low is 90°F, the thermal gradient between body and air is less than nine degrees. The body cannot shed heat fast enough to recover. It enters the following morning carrying the previous day's thermal load, and then adds more.

This cumulative load is the mechanism behind a well-documented pattern in desert heat mortality: deaths tend to cluster not at the peak day of a heat event but in the days after, when repeated failure to cool has exhausted the body's compensatory capacity. Phoenix's heat-mortality data from the Maricopa County heat surveillance record shows this clustering consistently [3].

Drug interactions

Several common drug classes actively suppress the body's thermal response. Diuretics — prescribed for hypertension, heart failure, and kidney disease — reduce circulating fluid volume, limiting the sweat output that is the body's primary active cooling mechanism. Beta-blockers, also prescribed for hypertension and arrhythmias, suppress the heart-rate increase that drives blood to the skin's surface during heat stress, reducing the body's ability to radiate heat. Antipsychotics, including both first-generation phenothiazines and several newer agents, interfere with hypothalamic temperature regulation directly, blunting the brain's awareness of thermal danger [1].

Patients on any of these medications face elevated heat-illness risk at ambient temperatures that would be manageable for someone not taking them. For these patients, the overnight low matters more than the daytime high. The daytime high is the moment the emergency begins; the overnight low is the reason the emergency began before they woke up.

Device-dependent care

The paper's Monday account of home medical device users established the Northeast exposure during the dome's prior phase: approximately 900,000 outages in New Jersey alone left an estimated three million device-dependent patients without powered equipment. The mechanism transfers to Phoenix. Oxygen concentrators, home ventilators, infusion pumps, insulin refrigeration, and CPAP machines all require electricity. Phoenix's desert-Southwest grid has different stress points than the Northeast, but the underlying care dependency is the same: the equipment must run or the patient is in danger.

The Salvation Army opened ten cooling stations across the Phoenix metro for the duration of the warning [1]. That is a coping inventory. It is not a risk measure.

The indoor death pattern

Twenty-nine heat deaths in New Jersey were found indoors — in apartments that had absorbed daytime heat through walls that did not cool overnight. Phoenix's predominant housing stock is stucco-on-frame construction, which absorbs radiant heat from direct sun during the day and releases it slowly through the night. An un-air-conditioned apartment that absorbs 115-degree daytime heat and sits in 90-degree overnight air does not approach comfortable temperatures at any point in a 24-hour cycle. The wall itself becomes a heat source [2].

The Maricopa County heat-death pace in 2026 is running ahead of 2025 — itself a record year — according to county surveillance data [3]. The county maintains a public heat-report dashboard updated weekly.

Who should act

Four categories of Phoenix-area residents carry elevated risk this week regardless of indoor air conditioning status.

People on diuretics, beta-blockers, or antipsychotic medications: Contact your prescribing physician today about heat-day protocols. For some medications, temporary dose adjustment is appropriate under physician guidance; for others, it is not. Do not self-adjust. Know that your medication changes your thermal risk profile.

Home medical equipment users: Confirm your equipment provider's emergency contact number and whether you are registered with your utility under a medical baseline rate or medical priority restoration program. Know your equipment's battery backup duration. If you lose power, call your provider before calling 911 unless you are already symptomatic.

Older adults in pre-1980 construction without central air conditioning: A box fan circulating 90-degree air provides no cooling benefit when indoor temperatures already exceed that level. A cooling station is the medically correct choice. Maricopa County 211 provides transportation referrals.

Residents without those specific risk factors in air-conditioned spaces: The acute risk is power outage. Know where the nearest cooling station is before you need it [1].

The daytime high of 115°F is the number on the banner. The overnight low of 90°F is the number that will determine whether this warning period costs lives. Phoenix will record near-record overnight lows through Thursday [2]. Bodies that cannot cool during those hours enter each day already compromised. The cooling stations are a response to that failure. The overnight low is the thing the cooling stations are trying to compensate for.

-- NORA WHITFIELD, Chicago