Your body warns you
before it's serious.
Atsusa watches your heart rate against real heat and humidity, personalised to you, and tells you the moment risk starts climbing. Not after.
For anyone who runs in heat: a first 5k, a marathon, a treadmill in a warm gym, or a slow jog on holiday. Heat illness doesn't check your pace.
Needs an iPhone + Apple Watch. The watch reads your heart rate live, so both are required.
Testing now, on wrists in about a month. Heatwaves don't wait for release dates, so testers get it first.
Why I'm building this
- 82 kg / 181 lb
I'm Dave, 51, and 168 cm (5 ft 6 in). Four years ago I stopped drinking. I was 82 kg (181 lb), very unhealthy, and after falling down and being in a very bad way, I decided to sort my life out.
- Zone 2
I started on an exercise bike in the shed, building my zone 2. I read about metabolic health and got tips from my cousin, who was getting into it after a friend was diagnosed with bowel cancer.
- Gym
I joined the gym, started lifting weights and experimenting with ketosis. Treadmill, spinning, yoga, Xcore, push, pull, legs, 10k steps a day. The weight slowly dropped off.
- 80 min
A 10 mile (16 km) run from Amsterdam to Zaandam, in 80 minutes.
- 19:29
Then I trained for a 5k (3.1 mi), done in 00:19:29.
- 10k · 45 min
Then it was time for a half marathon, 21.1 km (13.1 mi). I was fine for the first 10k (6.2 mi), 45 minutes in.
- Empty
Then I was empty, purely down to the weather conditions. I pushed on. And on.
- 01:40:00
Little did I know I was slipping away into a heat-exhausted state. My head felt like a wrecking ball wanting to drop backwards. Colour drained from my face. I was no longer able to sweat. I didn't notice anything until I crossed the line.
- 40.7°C / 105.3°F
If Anton and the rest of the medical crew hadn't been there, I wouldn't be typing this today. My temp was 40.7°C (105.3°F). I felt drunk, ironically. I should have been unconscious. They cooled me down and, well, saved my life. I'm sure of it.
At the start. Relaxed, colour in my face.
Colour gone, eyes glazed, my neck struggling to hold my head upright. This is what heat illness actually looks like from the outside.
At that same event, a 22-year-old student collapsed and died. That is the reason this can't just stay my story.
Meet Atsusa: the warning I didn't get.Dave
Heat illness arrives before you notice it.
Your body sends signals. The danger is that by the time you read them, you've already crossed into a state where reading them is hard. These are the warning signs I didn't feel until I was at 40.7°C (105.3°F).
WBGT climbs without feeling obvious. Humidity especially steals your cooling.
Sweat stops before you feel bad. The absence is the warning you miss.
Pushing on feels reasonable. It isn't. The decision centre goes first.
Pale, clammy skin is a late sign that your body is losing the fight.
Built from a race that nearly ended badly.
Anton and the race medical crew cooled me down and, I'm certain, saved my life. Atsusa exists so the warning arrives earlier than the medical tent.
My core temperature at the finish line of a half marathon.
A student collapsed and died at that same event. Same day, same heat.
I felt fine at 10k. By the line I couldn't sweat and my head wouldn't stay up.
Thresholds are built on real-feel heat and your own personal ceiling, not averages.
No reviews or ratings here yet. User testing is only just starting. When runners and coaches have used Atsusa in real heat, their words go in this section, and not a word before then.
Three steps between you and the warning.
Your thresholds are calculated from your actual max heart rate, tested or estimated via the Tanaka formula (207 - 0.7 x age). For Dave, at 51, that's 171 bpm.
Atsusa reads Wet-Bulb Globe Temperature, the real-feel measure that accounts for heat and humidity, and monitors your heart rate against it throughout your run.
As risk climbs, alerts escalate through five tiers and arrive as haptics you'll feel mid-stride: escalating triple-tap for warning, continuous for critical.
Five tiers, one job: tell you before it's too late.
Risk isn't binary. Atsusa escalates through five tiers as your heart rate climbs toward your ceiling, each with a clear instruction and a haptic pattern you'll recognise instantly. The watch does the sensing and the alerting; the phone holds the detail.
Keep going.
No action needed.
See where you're at at the flick of your wrist.
One number, one word, one ring. Alerts take the whole face and use the Digital Crown haptic click pattern: escalating triple-tap for warning, continuous for critical.
Atsusa needs both devices. The watch does the sensing, the phone does the thinking. That's why an iPhone and an Apple Watch are required.
A pre-run safety check.
Atsusa won't start monitoring until you've confirmed the basics. It's not nagging. It's the last clear-headed moment before you set off.
If you stop responding, Atsusa doesn't.
If a Critical alert goes unanswered, Atsusa enters Emergency mode automatically. It contacts your chosen person, a partner, a coach or a friend, and shares your location. You always have eight seconds to cancel with a single tap.
- Auto-escalates after unanswered Critical alerts
- Shares live location with your emergency contact
- 8-second countdown you can cancel anytime
- One-tap “I'm ok, cancel alert” escape
What testers ask before they sign up.
No records have ever been broken in warmer conditions. Run smart. Run safe.
Free while we're testing. Built to be forgotten, until it matters.
Testers pay nothing, and rounds are kept small so every piece of feedback gets read. Join now and you're in the first group.
Private testing rounds, small groups, free.
First tester builds land on real runs in real heat.
App Store, once the alerts are proven.
Requires an iPhone + Apple Watch. No cost to testers.