Why a 9°F Indoor-Outdoor Gap Is the Safe Line
When running AC in summer or heat in winter, a large gap between indoor and outdoor temperature forces your autonomic nervous system to work overtime adjusting to the swing. That can trigger so-called "AC sickness" symptoms — headaches, muscle aches, digestive discomfort, chronic fatigue — or raise heat shock risk from rapid blood vessel constriction and dilation. Experts generally recommend keeping the gap within about 9°F.
This calculator takes outdoor temperature and your indoor set temperature, calculates the gap, and rates the AC sickness / heat shock risk on a four-level scale. The recommended indoor range shown alongside it helps you set a cooling temperature that won't harm your health even on a scorching day. Managing this gap matters even more for older adults or anyone with cardiovascular conditions.
The same logic applies in winter — a large gap causes the same rapid vessel constriction and dilation, so keeping it within roughly 9-14°F is the safer range, similar to summer. When moving between environments with a big temperature difference, layering a jacket to create a buffer zone also helps.
Frequently Asked Questions
A gap of about 9°F or less is generally recommended. Beyond that, the risk of AC sickness or heat shock from a sudden temperature swing increases.
A large gap forces your autonomic nervous system to work overtime adjusting to the swing, which can trigger headaches, muscle aches, and digestive discomfort.
In winter, a large gap causes rapid blood vessel constriction and dilation that raises heat shock risk, so keeping the gap within about 9-14°F is safe, similar to summer.