News & Trends

The US Open put a number on dangerous heat. The guidance for everyone else mostly won't.

By the US Health Digest editorial team · Published August 26, 2026 · Every claim linked to its primary source

The 2026 US Open is underway at Flushing Meadows — August 23 to September 13, with the singles main draws starting Sunday, August 30, per Tennis Majors. Coverage is framing it around the ATP Tour's new heat rule: The Weather Channel's explainer is headlined "How the new heat rule will change the US Open this year" and applies the ATP thresholds "during the first two sets of a best-of-five match." That is wrong twice over. ESPN, reporting with the Associated Press, states flatly that "Grand Slam tournaments set their own heat policies," and the ATP rule it describes applies to best-of-three matches. What is worth your attention isn't the sport. It's that a tournament measures heat stress on court every 30 minutes and acts at a defined number — while the guidance aimed at you, on the same afternoon in the same city, is "stay hydrated."

What is actually true about the rule

The ATP Board's approval of a new heat policy was announced on December 15, 2025, effective from the 2026 season. Per ESPN's report with the AP: when Wet Bulb Globe Temperature "reaches at least 30.1 degrees Celsius (about 86.2 Fahrenheit) in one of the opening two sets of a best-of-three match, either player is allowed to request a 10-minute suspension of play. If the WBGT exceeds 32.2 degrees Celsius (about 90 Fahrenheit), the match will be halted." The WTA has had a heat rule since 1992; the men's tour moved only after players called for one during the humidity at the 2025 Shanghai Masters.

Three things follow that most coverage skips. This is the fourth major of the 2026 season, not the first anything. The ATP's rule applies to ATP Tour events; per ESPN and the AP, "The US Open, French Open and Wimbledon also have rules based on WBGT readings," as does Olympic tennis. And the tournament's own policy is older: Tennis.com's Peter Bodo reported in August 2025 that "US Open officials first formulated an official heat policy in 2018," keyed to a Heat Stress Index reading of 90°F and 50 percent humidity; the USTA added WBGT as a second guideline in 2024. CNN reported at the time that 2018 was "the first time in the event's history" such a policy was used, and that it "allowed men to take a 10-minute break between the third and fourth sets if a player requested it." The same report cuts against the tidy version: the USTA's Chris Widmaier told CNN that week, "I don't believe that we will have a concrete, written policy, if you will, on the extreme heat for the men until after this US Open as we see what is happening," and "We will be doing this on a case-by-case basis." All of that is those two outlets' account: usopen.org and the ITF and ATP rulebooks all refused our requests, so we have not read the tournament's current policy text first-hand. What survives the caveat is still narrower than "first Grand Slam under a new rule": a threshold was written down at this tournament years before 2026, and what changed this season is that the men's tour adopted the same index.

What 30.1 actually measures — and why it isn't the temperature

Wet Bulb Globe Temperature is not air temperature and not the "feels like" number on your phone. It is read off three instruments at once — "wet globe for humidity, black globe for solar factor, and dry bulb for the ambient temperature," per the National Weather Service's WBGT Informational Guide. NWS's Heat Forecast Tools page puts the contrast plainly: WBGT "uses temperature, humidity, wind, solar radiation, and other weather parameters" and is "a particularly effective indicator of heat stress for active populations such as outdoor workers and athletes." The heat index, by contrast, "is a measure of how hot it really feels when relative humidity is factored in with the actual air temperature" — and NWS warns that "since heat index values were devised for shady, light wind conditions, exposure to full sunshine can increase heat index values by up to 15°F."

So a WBGT of 30.1°C does not mean it is 86°F outside. It is a composite of sun, wind, humidity and air temperature, measured where the activity happens — which is why, NWS notes, it has "been used for decades in many military agencies," by OSHA and by "organizations that facilitate marathons," and is "utilized by athletic programs in many school districts."

And the tournament measures it: Bodo reported that in 2024 "the official US Open tournament medical team from Mt. Sinai Hospital monitored WBGT every 30 minutes," though conditions "never reached the tipping point that allows for emergency heat mitigation measures that include additional breaks, the use of cooling devices and roof closures." He also notes what the tournament has not done: "like the Australian Open, the US Open has avoided adopting a policy that demands mandatory suspension of matches when certain heat and humidity conditions occur." A measured threshold with a discretionary trigger is still a threshold — more than most people get.

Three institutions, three thresholds — and they are not the same measure

Read the "measure" column before comparing any two rows: three rows below say 90°F, and none of them mean the same thing by it. None of these numbers is an enforceable federal standard.

The OSHA rows repay a second read. Their WBGT triggers are not fixed numbers at all — they are tied to the Recommended Alert Limit and Recommended Exposure Limit in NIOSH Publication 2016-106, which vary with workload and acclimatization: a roofer's trigger depends on how hard and how long they have been working.

And that proposal is still a proposal. The Labor Department's regulatory agenda of August 14, 2026 still lists Heat Injury and Illness Prevention (RIN 1218-AD39) in the Proposed Rule Stage, with a supplemental proposal projected for December 2026 and final action for October 2027. Two years after it published, there is no federal heat standard in force for American workers.

What the guidance for everyone else actually says

What follows is not advice from us; it is what the agencies publish, quoted so you can check it. CDC's "About Heat and Your Health" page, updated July 20, 2026, keys its advice to the HeatRisk forecast rather than to a temperature: "If you are outside, especially for a long time, and the HeatRisk is red or above," you can "stay in the shade as much as possible," "take breaks when you can," and shift activity "during the coolest parts of the day or evening, if possible."

Indoors, one CDC instruction is unusually specific: "Use fans, but only if indoor temperatures are less than 90°F. In temperatures above 90°F, a fan can increase body temperature." On fluids, CDC says to "carry a water bottle," to "consider limiting beverages high in sugars and sodium, caffeine, and alcohol," and to "check your urine color" — but sets no quantity target for the public. The symptoms it wants people to recognize: "muscle cramping, unusually heavy sweating, shortness of breath, dizziness, headaches, weakness, nausea." CDC's NIOSH page names the syndromes behind them.

The NWS HeatRisk tool CDC points to runs 0 to 4, weighing how unusual the heat is for the season and how long it lasts, day and night; NWS labels it experimental. Note the shape of this. The tournament threshold produces an action at a number; the public guidance produces a category, a colour and a suggestion. That gap is deliberate — NWS says its criteria vary because "residents of Florida are much more prepared for 90°F+ weather than residents in Alaska" — but it is worth naming.

Who CDC says is at higher risk

CDC's People at Increased Risk for Heat-Related Illness page names seven groups: adults 65 and over; infants and children; people with chronic conditions; people without air conditioning; athletes, because "people who exercise in heat are more likely to become dehydrated and get heat-related illness"; outdoor workers; and people who are pregnant. CDC's consumer page adds asthma and heart disease.

On medication, CDC's public guidance is two sentences long and both matter: "Many medicines can make you dehydrated or overheated on hot days. Also, some medicines need to be kept out of hot places." Immediately followed by: "Don't stop or change your medicines until you talk to your doctor."

One narrow note if you take a GLP-1

GLP-1 medications commonly cause nausea, vomiting and diarrhea and reduce food and fluid intake, and the Wegovy (semaglutide) prescribing information on DailyMed states in Section 5.5 that the majority of postmarketing reports of acute kidney injury "occurred in patients who experienced gastrointestinal adverse reactions leading to dehydration such as nausea, vomiting, or diarrhea." Dehydration is also the pathway heat guidance is built around. That overlap is a documented mechanism, not a measured heat risk — and it is what CDC means by a medicine that "can make you dehydrated."

We have covered this in full — including what CDC's clinician guidance does and does not say about this drug class, and why no study has quantified the interaction — in GLP-1 medication in a record-hot summer, with the trial side-effect numbers in GLP-1 side effects in women, nausea and food in eating on a GLP-1, and prescriber questions in our GLP-1 FAQ for women. Nothing here is a reason to start, stop or change a medication; CDC's instruction on that is quoted above.

The bottom line

A tennis tournament decided "hot" was too vague to be useful, so it bought instruments, defined a number, had a hospital team read it every half hour, and wrote down what happens next. For everyone who works, walks, coaches, delivers or plays outdoors, the equivalent numbers exist on paper — a heat index of 80°F and 90°F, in a Federal Register document from August 2024 — and have sat in the Proposed Rule Stage ever since. This tournament ends September 13. That rulemaking is scheduled to finish in October 2027.