According to numerous studies cited in the Centers for Disease Control's (CDC's) report "Review of the Effects of Noise on Man" and Toronto Public Health's report "Health Effects of Noise," long-term occupational exposure to noise -- especially high-frequency noise -- leads to hearing loss. The studies cited in these reports also suggest that long-term exposure to noise can lead to stress-related conditions such as hypertension, and endocrine and neurological responses, but the exact correlation is unclear and there's great individual variation in tolerance to noise. Some studies have also shown a relationship between high noise levels and decreased performance at school or in the workplace.
Long-term exposure to loud noise affecting hearing loss has been known anecdotally for centuries, but it was not until the latter half of the 20th century that science could conclusively prove it. The EPA agrees that prolonged exposure to noise above 55 to 60 decibels can cause hearing damage. A 1981 EPA study estimated that 2.2 million Americans have been exposed to industrial noise greater than 60 decibels, and that 9.1 million are exposed to industrial noise levels above 55 decibels. This CDC report and the "Health Effects of Noise Technical Report" regarding noise at the Los Angeles International Airport also discuss studies demonstrating that exposure to loud high-frequency noise leads to hearing loss in most people.
Physiologic Stress Responses
The CDC and Toronto Public Health reports just referenced present several studies linking exposure to loud noise as well as a variety of physiologic stress responses in people. An increase in blood pressure and heart rate are typical, and there's mounting evidence of a broad range of endocrine and hormonal neurological responses to loud noise, especially immediately after hearing it. While some older studies show that people do get used to loud noises and the levels of stress responses drop significantly, these responses don't completely disappear. Other research has shown that at least some people continue to have elevated blood pressure and higher levels of stress hormones with long-term exposure to intermittent loud sounds.
Noise Lowers Ability to Concentrate
According to the CDC, there's strong evidence that noise interferes with the ability to concentrate. However, the effects of noise on concentration are highly variable based on the type of noise and the individual. In general, loud intermittent noise is more interruptive than loud steady noise. A 1995 study showed a "hangover effect" from noise when performing tasks requiring higher cognitive performance, according to the city of Montreal. Research has also shown that noisy workplaces are typically less productive and have higher accident rates than quieter workplaces.
Interference with Communication
Noise interferes with communication. Even moderate noise levels require shouting at distances of 10 feet or more, and noise makes normal verbal communication almost impossible at longer distances. Although many noisy workplaces mitigate this problem with radio headsets, speakers or other technology, there's still some impairment to overall communication. In the long run, poor communication results in less productivity and more accidents.