It should come as no surprise that rats exposed to a buzzer sounding for six out of every 30 seconds, seven hours a day for 35 days suffer from high blood pressure. There is some sign of habituation over time, but these buzzer-rats are still darting back and forth across their cages by day 35, while rats in quieter cages have markedly lower blood pressure and tend not to pace so nervously.

For humans, six hours of exposure to 90-decibel sound significantly elevates the heart rate and leaves it there up to an hour after the noise is gone. Nearly every significant study looking for a link between exposure to noise and risk of heart attack has found one. In 2005, research in Berlin hospitals looking at more than 4,000 cases (half of them heart attacks) revealed that people subjected to loud environments are at a 50 percent greater risk of having a heart attack.

Among school kids, the effect of noise shows up in the form of learning disabilities.