Decide the switch before you leave, and decide it in numbers. The EPA's air quality index turns orange at 101, where the category becomes unhealthy for sensitive groups, with 51 to 100 moderate and the plain unhealthy band starting at 151; the National Weather Service warns that full sunshine can raise the heat index by up to 15°F (8°C), so a shaded forecast is not the number you will be walking in. Pick your thresholds at the kitchen table, not at the trailhead. The condition that changes the answer is what the outing was for: when the place itself was the point, the honest alternative is another day, not another building.
Set the switch in numbers before you leave
Write one sentence: "We move indoors if ___." Fill it with a figure you can look up in thirty seconds.
Air is the easiest. The index runs 0 to 50 good, 51 to 100 moderate, 101 to 150 unhealthy for sensitive groups, 151 to 200 unhealthy, and each band carries a color, so a glance settles it. Heat needs one more step, because the heat index is calculated for shade: the Weather Service's own example puts 96°F (36°C) with 65 percent humidity at a heat index of 121°F (49°C), and full sun adds up to another 15°F (8°C) on top of whatever the chart says. A route with no tree cover is a different forecast from the one on your phone.
Then say what the outing is for, in one more sentence. Conversation, an errand, movement, or the place. Only the first three have indoor equivalents.
Scroll sideways to see the full table →
| Outcome | What has to be true | What you send |
|---|---|---|
| Plan A, outdoors | Index at or below your number, heat index within your limit, route open | Meeting point and turnaround |
| Plan B, indoors | Building open, activity allowed, cost known, entrance checked | New entrance, changed activity, any cost |
| Reschedule | The place was the point, or nobody wants the substitute | A cancellation before anyone sets out |
Cancellation is the third ordinary outcome, not an embarrassment. It is what the checking was for.
Confirm that the room allows the activity
A building open to the public is not a building that permits your plan. The ADA's twelve categories of public accommodation are a useful map here: a museum, library or gallery is category eight, a park is category nine, and a gymnasium, health spa or other place of exercise or recreation is category twelve. A library is a place to sit and talk quietly; it is not a walking track, and a corridor is not a substitute route because it is indoors and long.
So ask the operator three things: whether the room is open at your hour, whether your activity is allowed in it, and what it costs. Then check the entrance the way you would outdoors — the ADA Standards put an accessible route at 36 inches (915 mm) wide, a doorway at 32 inches (815 mm) clear, and a ramp at no steeper than 1:12 — because Plan B usually arrives at a door nobody has looked at yet.
Send the message before anyone travels
An example, with the numbers filled in. Two people plan a garden walk mainly to catch up. The morning index reads 118, which is inside the orange band, so the switch trips. Plan B is a community lounge, open from 9, seating unrestricted, no charge, step-free from the street side; the walk becomes a seated hour and the purpose survives. Had the garden itself been the reason for the day, the same 118 would have produced a different message: not a lounge, but next Saturday.
Notice which fact did the work. Not the weather, and not a preference — a published number with a published threshold, agreed in advance by two people who were not yet standing outside disagreeing about how bad it looked.
Write the message before you need it. "Plan A is off. Plan B is ___ at ___, with ___. If that does not work, let's reschedule." Send it before anyone travels. Keeping an invitation alive does not require anyone to accept a replacement, and a clear ending leaves the next invitation possible. If the walk outdoors is still on, the return is the part worth planning first, and if the indoor version has to be free, a place to meet that does not require a purchase is its own question.
Sources and further reading
- Air Quality Index (AQI) BasicsAccessed 2026-09-17
- Heat Forecast ToolsAccessed 2026-09-17
- 28 CFR 36.104 DefinitionsAccessed 2026-09-17
- Chapter 4: Accessible RoutesAccessed 2026-09-17