WASHINGTON — An intense and far-reaching heat wave is affecting communities across the United States, exposing millions of people to potentially hazardous temperatures and providing limited relief after sunset. Forecasters say more than 90 local temperature records could be matched or surpassed through Wednesday. Many of these are expected to involve record-high overnight lows, which can be especially harmful because the body has fewer opportunities to cool down and recover. The extreme conditions are being driven by a heat dome—a strong region of high pressure that holds hot air close to the surface and prevents cooler weather systems from moving through. At its greatest extent, the system could influence nearly two-thirds of the continental United States. In several areas, temperatures are forecast to climb between 15 and 25 degrees above seasonal averages. Parts of the West, central states, and northern Plains have already experienced readings in the triple digits, while the oppressive conditions continue spreading eastward. Associated Press Warm nights intensify the health threat The absence of meaningful nighttime cooling is one of the greatest concerns surrounding this heat wave. Temperatures may remain above 80 degrees overnight in cities including Miami, Tampa, Fort Lauderdale, Galveston, and Charleston. Even typically cooler communities such as Fargo, International Falls, and Portland, Maine, could experience nighttime temperatures above 70 degrees. Extended exposure can place continuous stress on the body, particularly the heart and kidneys. When indoor and outdoor temperatures remain high throughout the night, people may begin the following day without having recovered from the previous day’s heat. The health consequences can build gradually. A person who initially appears unaffected may become dehydrated, exhausted, or seriously ill after several days without access to a properly cooled environment. Those facing the highest risk include older adults, infants, pregnant people, outdoor laborers, and individuals with chronic medical conditions. Residents without air conditioning, people experiencing homelessness, and those who live alone may also be especially vulnerable. Recognizing heat-related illness Heat exhaustion usually develops when heavy sweating causes the body to lose excessive amounts of water and salt. Symptoms may include: Profuse sweating Fatigue or weakness Dizziness or headache Muscle cramps Nausea or vomiting Pale, cool, or damp skin A rapid but weak pulse Anyone showing these signs should stop physical activity, move into a cooler area, and sip water if they can do so safely. Wet towels, cool showers, or baths can help lower body temperature. Medical assistance should be requested if the person begins vomiting, deteriorates, or does not improve within an hour. Heat stroke is a life-threatening emergency. Possible symptoms include confusion, fainting, severe headache, nausea, hot or reddened skin, and a body temperature of at least 103 degrees. If heat stroke is suspected, call 911 immediately. Begin cooling the person while waiting for emergency assistance, but avoid giving drinks to anyone confused, unconscious, or unable to swallow safely. CDC heat-illness guidance Pressure mounts on workers, utilities, and communities High temperatures are driving up electricity use as air conditioners operate for longer periods. During peak-demand hours, utility providers may ask customers to conserve energy to reduce strain on power systems and lower the possibility of outages. People employed outdoors or in hot indoor environments face additional danger. Construction crews, agricultural workers, delivery drivers, landscapers, and road workers may need more frequent rest periods, reliable access to drinking water, and shaded or air-conditioned recovery areas. Public facilities such as libraries, community centers, and recreation buildings may operate as cooling centers. Residents seeking assistance should consult local government announcements for locations and operating hours. The heat may also worsen wildfire conditions in dry sections of the West and Rocky Mountains. Lightning from thunderstorms that produce little rain could ignite new fires in areas where vegetation is already dry. A warmer climate raises the stakes Heat domes are naturally occurring weather systems. However, climate scientists say rising global temperatures are making many heat waves more severe, widespread, and persistent. Because average temperatures are higher, extreme events now develop against an already warmer background. A strengthening El Niño in the Pacific could influence weather later in the year, but specialists say it is too newly formed to be considered the main driver of this particular heat event. Unlike storms that leave visible damage, extreme heat can appear less threatening. Its effects are often gradual and may not become evident until several days after exposure. Despite its quieter nature, heat remains one of the most dangerous weather hazards in the country. Recommended safety measures People in affected communities are encouraged to: Drink water throughout the day, even before becoming thirsty. Postpone strenuous outdoor work or exercise during peak afternoon heat. Choose loose, lightweight, and light-colored clothing. Spend several hours in an air-conditioned location when possible. Contact older relatives, neighbors, and anyone living alone. Keep children and animals out of parked vehicles at all times. Ensure pets have sufficient water and protection from direct sunlight. Store temperature-sensitive medicines according to their instructions. Electric fans may provide comfort, but they are not always adequate when indoor temperatures become extremely high. The CDC warns that fans can sometimes add to heat exposure when temperatures exceed 90 degrees. Cooling the skin with water or entering an air-conditioned space offers better protection. CDC heat-and-health information Outlook Dangerous temperatures are expected to persist across numerous states through at least midweek, and some locations may remain unusually hot for a longer period. Because conditions will vary between regions, residents should monitor forecasts and warnings issued by their local National Weather Service office. The Genuine Update takeaway: This event is dangerous not simply because of its daytime highs, but because the heat is widespread, long-lasting, and continuing through the night. Drinking enough water is essential, but spending time in a properly cooled environment provides the strongest protection.