Effects of Water and Ambient Air Temperatures on Human Diving Bradycardia
Authors: Erika Schagatay, Boris Holm
DOI / Source: https://doi.org/10.1007/BF00262802
Date: 01 August 1995
Reading level: Beginner
Why This Matters for Freedivers
This paper explains a common real-world surprise: your dive reflex can feel stronger in “cool” water after you’ve been warm in the sun, and weaker when you’re already cold before you start. It’s not just the water temperature that matters—what your body experienced before the dive changes how strongly your heart rate drops.
Synopsis
Freedivers often say cold water on the face boosts the dive reflex. That’s true—but this study shows the story is a bit more specific: the water temperature and the air temperature you were in beforehand interact, and they can pull the dive reflex in different directions.
Twenty-three healthy volunteers did three separate test days. Before each test, they sat for 60 minutes in a room set to one of three air temperatures: 10°C, 20°C, or 30°C. After that, they performed a maximal breath-hold in air (no immersion) and then a set of maximal breath-holds with face immersion in water at 10°C, 20°C, or 30°C. Heart rate was recorded throughout. The researchers focused on how much the heart rate dropped (bradycardia), especially during the last part of each breath-hold when the response is fully established.
A few things stood out clearly:
- Apnea alone caused bradycardia at all air temperatures, but the differences between air temperatures weren’t dramatic.
- Apnea + face immersion produced a much stronger bradycardia than apnea in air (the classic “diving” effect).
- Water temperature mattered, but not in a simple “colder water always equals bigger bradycardia” way unless you also consider what happened before the test.
- Ambient air temperature mattered too, and it pushed the response in the opposite direction: in general, warmer air exposure led to a bigger bradycardia during the subsequent face-immersion apnea.
The most pronounced bradycardia happened when subjects had been sitting in warm air (30°C) and then immersed the face in cold water (10°C). The weakest response was the opposite combination: cold air (10°C) followed by warm water (30°C), where the bradycardia could be so small that it barely differed from resting.
Why would warm air make the dive reflex stronger. The authors’ explanation is practical and intuitive. In warm air, your body tends to be more vasodilated and your heart rate is a bit higher. When you suddenly trigger the diving response, the shift to vasoconstriction and parasympathetic “brake” on the heart is larger—so the drop in heart rate is more pronounced. On top of that, temperature receptors respond strongly to rapid change (dynamic sensitivity). If you’re warm and then your face suddenly hits much colder water, the “shock” to the receptors is bigger, which can amplify the reflex. If you’re already cold, that cold-water stimulus is less dramatic.
The study also explains why different labs sometimes report conflicting results about “does water temperature affect diving bradycardia?” If a lab tests in a warm room versus a cool room, the same water temperatures can produce different patterns. Their takeaway is simple: ambient air temperature should always be reported in studies on the human diving response.
For freedivers, the practical message is: the dive reflex is not only about the water. The same face-immersion in the same water can feel very different depending on whether you were warm, neutral, or chilled beforehand.
Abstract
Upon apnoeic face immersion, humans develop a diving response resembling that found in diving mammals. There have been contradictory reports regarding the influence of water temperature on the magnitude of the resulting bradycardia. This study examined the influence of both water and ambient air temperatures on human diving bradycardia. A group of 23 volunteers performed three series of apnoeic episodes after 60-min exposure to air at temperatures of 10, 20 or 30°C. Heart rate (HR) was recorded during apnoea in air and apnoea with the face immersed in water of 10, 20 or 30°C, at each air temperature. Both air and water temperatures had significant effects on immersion bradycardia, but in opposite directions. Face immersion in cold water after exposure to a high ambient air temperature induced the most pronounced bradycardia. The range in which the response was correlated to water temperature differed depending on ambient air temperature. The study concludes that human bradycardia from apnoeic face immersion is inversely proportional to water temperature within a range determined by the ambient air temperature.