Fitbit Zip Ionic Faq page 7

If your device has an Exercise app, you can check your real-time heart rate and heart-rate zone during a
workout. Note that heart-rate tracking is disabled while swimming.
On Fitbit Ionic and Fitbit Versa series, the heart-rate value appears gray if your watch is searching for a
stronger reading.
Keep in mind that several factors can affect your heart rate, including air temperature, stress level,
physical body position, and medication use (source).
What are heart-rate zones?
Heart-rate zones can help you optimize your workout by targeting different training intensities.
Fitbit estimates your maximum heart rate with the common formula of 220 minus your age, and
calculates 3 heart-rate zones based on that number. Occasionally during intense exercise, you