Dear Doctor: My parents, who are in their late 70s, are both now using hearing aids. This makes me wonder -- when should a hearing test become part of an annual checkup?

Dear Reader: Our ears connect us to the world, and yet until something goes wrong, hearing is one of the senses we seem to take for granted.

It's a good idea to have a hearing test as a baseline when you're a relatively young adult so that if you develop a hearing issue later, you will have a useful comparison. You can talk to your family doctor about giving you a hearing test or seek out the help of a hearing professional, known as an audiologist.

Hearing loss usually happens so gradually that you may not even be aware of it. It's not until you run into problems such as frequently asking people to repeat themselves or cranking up the volume on your TV that you realize something has changed.

Hearing is a complex process. When you hear a sound, your brain is interpreting electrical signals that it receives via the auditory nerve. These signals originate in the delicate structures within your inner ear, which receive sounds -- that is, vibrations -- and turn them into nerve impulses.

It's important to identify hearing loss as soon as possible. Studies suggest a link between hearing loss and serious conditions such as depression or dementia. An early diagnosis and successful intervention can reduce symptoms of depression and help preserve cognitive function.

If you think you may have trouble with your hearing, you're not alone. About 15 percent of American adults report some form of hearing problem. Signs that you could be experiencing hearing loss may include:

-- Frequently asking people to repeat themselves;

-- Having trouble pinpointing the source or direction of a loud noise;

-- Trouble hearing telephone conversations;

-- Being told the volume of your TV or radio is excessively loud;

-- Difficulty understanding a person who isn't facing you as he speaks;

-- Struggling to hear conversations in noisy environments, like a restaurant;

-- Difficulty understanding high-pitched voices;

-- Being told by others that your hearing seems impaired.

If you've experienced these situations, a hearing test is a good idea. It will reveal whether you have hearing loss in either ear, and pinpoint the type of hearing loss and to what degree it has progressed.

There are several types of hearing tests.

A physical exam with an instrument called an otoscope reveals any problems in your ear canal or eardrum. Additional tests include a pure tone test, which reveals how well you can hear -- you guessed it -- a variety of pure tones.

A speech test evaluates your ability to understand the spoken word. Tympanometry tests reveal any problems in the middle ear and evaluate the mobility of your eardrum.

These hearing tests are painless. Taken together, the results offer a detailed picture of your hearing, known as an audiogram. And if a problem should be uncovered, your audiogram gives you and your doctor the information needed to move forward.

(Eve Glazier, M.D., MBA, is an internist and assistant professor of medicine at UCLA Health. Elizabeth Ko, M.D., is an internist and primary care physician at UCLA Health.)

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095.)

Dear Doctor: We live near a freeway with a lot of traffic. Do I need to be worried about my kids developing asthma?

Dear Reader: This is a valid question. Cars release a number of small particles into the air that can irritate the lungs. This particulate matter, in addition to nitric oxide and black carbon, has been associated with asthma. Children are more vulnerable to the effects of pollution because their lungs and immune system are still developing.

An analysis of many studies, published in the journal Allergy, found a significant association between traffic-related air pollution and asthma. The association was seen as early as the age of 1 and increased as the child became older. The rise in asthma as kids get older seems to be related both to structural changes that pollution creates in the lungs and also to the extent of time that older children spend outside compared to younger children.

One other important aspect of asthma is that it can be exacerbated by stress. Noise pollution and the danger of cars in close proximity may worsen asthma symptoms in children because they can create a more stressful environment. This is an important factor if your child already has asthma.

A recent study in Minnesota evaluated the number of asthma cases in the emergency room, in the hospital and in a clinical setting. The researchers then looked at the proximity of people to a major source of traffic. After taking poverty out of the equation, which is in itself a risk factor for asthma, the researchers found that, as traffic density increased, so did the number of asthma exacerbations, or worsening of asthma symptoms. In this study, for every 10 percent increase in traffic density, there was a 15 percent increase in asthma-related visits to the emergency room, hospital or outpatient clinic.

Clearly, if you have a child with asthma, moving him or her away from traffic would indeed reduce the chance of a severe asthma attack.

I understand your concern. It is also a societal concern. Despite the evidence linking pollution to asthma, the levels of pollutants that are associated with an increased risk of asthma are well below the guidelines from the World Health Organization.

This doesn't mean that if you live near a freeway, you have to move. The majority of children who live near a freeway do not get asthma. But if your child is already showing signs of asthma at a young age, then moving to an area with fewer pollutants is perhaps something you should consider, if it is at all possible.

It took many years for our society to accept the ills of secondhand smoke from cigarettes. When it did, smoking was then banned from many public places, even casinos. It is more difficult to ban housing near freeways or heavily trafficked areas, especially when that housing already exists.

(Robert Ashley, M.D., is an internist and assistant professor of medicine at the University of California, Los Angeles.)

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095.)

Dear Doctor: We're finally winning the battle against sugary soft drinks in our home, but a family friend insists that the sparkling water our kids are drinking instead is bad for their bones and teeth. Is this really true?

Dear Reader: First, congratulations on weaning your family off of soft drinks. One out of every three Americans drinks at least one (and often more) sugar-packed soda or other sweetened beverage every day. This puts them at increased risk for serious health problems such as obesity, Type-2 diabetes and even heart disease.

Although in a perfect world we would all stick to plain water to get the hydration we need, the truth is that can get boring. It's no surprise, then, that sparkling water, with its fizzy bubbles and wide range of flavors, has become a go-to replacement for people who want to cut back on calorie-laden sodas.

Sparkling water, which typically has no calories, is made when carbon dioxide gas is dissolved in plain water, a process known as carbonation. This results not only in the bubbles we love, but also creates carbonic acid, which gives fizzy water a mildly tart flavor. (We'll get back to that in a minute.)

The concern voiced by your family friend, that drinking carbonated water weakens bones, quite possibly finds its roots in a study conducted in 2006. Known as the Framingham Osteoporosis Study, researchers tied the consumption of cola beverages to decreased density in the hip bones of older women.

But the operative word here turns out to be "cola." Women in the study who drank non-cola beverages did not exhibit increased bone loss. The researchers concluded that carbonation doesn't damage your bones.

As to the question of carbonation being bad for your teeth, we need to talk about the carbonic acid we mentioned earlier.

While it's true that the process of carbonation results in the creation of an acid, it's a very weak one. Plain bottled water has a neutral pH of 7. Carbonated water such as Perrier is only slightly more acidic, with a pH of 5.25. According to the American Dental Association (ADA), that's "minimally corrosive." Cranberry juice, by contrast, with a pH of 2.5, is considered by the ADA to be "extremely corrosive."

So the good news is that you and your family are on the right track with fizzy waters replacing sodas and other sweet drinks. Just be sure to always read the labels. If you see the words sucrose, glucose, fructose or corn syrup, you're holding a sugary drink.

And here's a bonus for weight watchers from a National Institutes of Health study: Young women who drank a glass of fizzy water reported feeling full and satiated.

Lastly, for those of you with kids who may have heard this urban legend -- eating the carbonated candy Pop Rocks and then drinking a soda will definitely not make your stomach explode!

(Eve Glazier, M.D., MBA, is an internist and assistant professor of medicine at UCLA Health. Elizabeth Ko, M.D., is an internist and primary care physician at UCLA Health.)

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095.)