Racket, din clamor, noise, whatever you want to call it, unwanted sound is America's most widespread nuisance. But noise is more than just a nuisance. It constitutes a real and present danger to people's health. Day and night, at home, at work, and at play, noise can produce serious physical and psychological stress. No one is immune to this stress. Though we seem to adjust to noise by ignoring it, the ear, in fact, never closes and the body still responds--sometimes with extreme tension, as to a strange sound in the night. The annoyance we feel when faced with noise is the most common outward symptom of the stress building up inside us. Indeed, because irritability is so apparent, legislators have made public annoyance the basis of many noise abatement programs. The more subtle and more serious health hazards associated with stress caused by noise traditionally have been given much less attention. Nevertheless, when we are annoyed or made irritable by noise, we should consider these symptoms fair warning that other thing may be happening to us, some of which may be damaging to our health. Of many health hazards to noise, hearing loss is the most clearly observable and measurable by health professionals. The other hazards are harder to pin down. For many of us, there may be a risk that exposure to the stress of noise increases susceptibility to disease and infection. The more susceptible among us may experience noise as a complicating factor in heart problems and other diseases. Noise that causes annoyance and irritability in health persons may have serious consequences for these already ill in mind or body. Noise affects us throughout our lives. For example, there are indications of effects on the unborn child when mothers are exposed to industrial and environmental noise. During infancy and childhood, youngsters exposed to high noise levels may have trouble falling asleep and obtaining necessary amounts of rest. Why, then, is there not greater alarm about these dangers? Perhaps it is because the link between noise and many disabilities or diseases has not yet been conclusively demonstrated. Perhaps it is because we tend to dismiss annoyance as a price to pay for living in the modern world. It may also be because we still think of hearing loss as only an occupational hazard. In Paragraph 1, the phrase 'immune to' is used to mean ______.