Science project

Hot Tubs and High Blood Pressure


  • Hot tub
  • Bathtub
  • Blood pressure monitor
  • Notebook
  • Pencil
  • Stopwatch
  • Towel


  1. Have your friend sit normally in a chair. Following the directions on the , place the blood pressure monitor on your friend’s arm and take her blood pressure.
  2. Record your friend’s systolic and diastolic blood pressure. Systolic blood pressure is the top number of your blood pressure reading. This measures the pressure in the arteries when the heart beats. The bottom number is your diastolic pressure, and this measures the pressure in the arteries when the heart is resting between beats.
  3. Write these measurements in your notebook. Get in the hot tub with your friend.
  4. As you enter the hot tub, set the stopwatch for fifteen minutes. After fifteen minutes, take your friend’s blood pressure. Use the towel to dry off her arm. Then, place the blood pressure monitor on her arm and take her blood pressure. Has it changed? Is it the same? Record your data in your notebook.
  5. To determine the percentage difference, divide the first reading by the second reading (do this for both systolic and diastolic blood pressure).
  6. If the number you get is larger than one, this means that your friend’s blood pressure has gone up in the hot tub. For example, 1.05 means that your friend’s blood pressure has gone up 5 percent (0.05).
  7. If you get a number that’s less than one, that means that your friend’s blood pressure has gone down. For example, 0.90 means that your friend’s blood pressure has gone down by 10 percent (0.10). Did your friend’s blood pressure go up or down? Why do you think this is?
  8. If you’d like, you can repeat the same experiment with several different friends. Take an average of your results to determine whether hot tubs have a tendency to increase blood pressure or lower it.


Is there really a connection between hot tubs and high blood pressure? Actually, your friend’s blood pressure will likely have dropped slightly (by around 5%) after fifteen minutes in the hot tub.


As your heart beats, it pushes blood through your arteries. This force causes pressure. Measuring blood pressure is one way to assess health. Sometimes, arteries can get clogged up with fatty deposits, restricting blood flow and increasing blood pressure. In cases where blood vessels become very obstructed, heart attacks can occur. Obstructed arteries can also damage organs (remember—organs need blood in order to function!). Some disorders also lead to low blood pressure. A nervous system difference called vasovagal syncope leads to a slower heart rate and wider blood vessels, which in turn can cause low blood pressure and fainting.

So what’s considered a normal blood pressure measurement? Your blood pressure is different at different times of day. Healthy blood pressure is usually around 120 mm HG or under for systolic (when your heart is beating). It should be 80 mm HG or under for diastolic (when your heart is between beats).

What can change your blood pressure? Lots of things! Activity changes your blood pressure. Standing up or sitting down can change your blood pressure temporarily. Stress can make your blood pressure go up, and sleep can make it go down.

Why do you sometimes feel dizzy after you get into the hot tub? In this experiment, your friend’s blood pressure went down due to vasodilation. Hot temperatures can make our muscles relax. This includes the muscles that control the expansion and contraction of our blood vessels. Dilating, or expanding, blood vessels form a wider “highway” for blood, lowering blood pressure. Orthostatic hypotension happens when blood pressure decreases after standing up or sitting down. When you combine this with the low blood pressure caused by the hot tub, this can explain why a person in a hot tub might feel dizzy.

Extend your experiment. What would happen if you asked your friends to sit in a bathtub of cold water? Would their blood pressure go up, go down, or stay the same? Find a brave friend and try it out.

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