Wednesday will be the most dangerous day yet in Greenwich's current heat wave, with temperatures expected to reach the upper 90s to low 100s, according to the town.

The Greenwich Department of Health issued a heat advisory Tuesday, warning that dangerously hot temperatures combined with high humidity will last through Thursday. Health Director Caroline Calderone Baisley urged residents to know the warning signs of heat illness and take precautions now. Connecticut's extreme hot weather protocol runs through 5 p.m. Wednesday, and the National Weather Service advisory continues until 9 p.m. Overnight temperatures aren't offering much relief either, with lows only dropping to the upper 70s and low 80s. Ground-level ozone may also climb into unhealthy territory during the peak heat.

Young children, adults over 65, people with chronic conditions like diabetes or heart and respiratory disease, outdoor workers, anyone without air conditioning, and people who are socially isolated all face higher risk. The health department is asking residents to check on elderly neighbors and anyone with medical or mental health conditions during the heat event.

Heat stroke is a medical emergency — call 911 immediately if you see confusion, a high body temperature, rapid pulse, headache, nausea, or trouble breathing. Skin may be hot and dry, or the person may still be sweating heavily.

Heat exhaustion looks different: heavy sweating, weakness, cool clammy skin, muscle cramps, dizziness, and a weak pulse. Move the person somewhere cool and offer small sips of water; seek medical help if vomiting continues.

One key reminder from the health department: if you're waiting until you feel thirsty to drink water, dehydration has already started. Skip caffeine, alcohol, and sugary drinks, and never leave a person or pet in a parked car — even with the windows cracked.

The Margenot Atrium at the Greenwich Public Safety Complex stays open 24/7 as the town's main cooling center. During regular hours, you can also cool off at Greenwich Town Hall, Greenwich Library and its Byram and Cos Cob branches, Perrot Memorial Library, and the Bendheim and Cohen civic centers. More options are listed at 211.ct.org.

Humidity drops Thursday and Friday, with highs easing back to the mid-to-high 80s by week's end. This marks the third significant heat event to hit Greenwich this summer, following episodes in mid-June and late June into early July.

Sign up for emergency weather alerts at GreenwichAlerts.org, or reach the Greenwich Department of Health at (203) 622-7836 with non-emergency questions.