Dear Doctor: With cooler weather here in Minnesota, our kids, who love swimming, are switching from the lake to the pool at our local Y. Just how dangerous is it to swim in a public pool? I keep hearing there's a bug that chlorine doesn't kill.

Dear Reader: Any time you share a common space, whether with one other person or a crowd, you run the risk of being exposed to whatever germs, viruses, parasites, bacteria, molds or other environmental hitchhikers they may have brought along. Unsurprisingly, this applies to the public swimming pool, where diarrhea is the most common recreational water illness. And while chlorine significantly reduces the potential health risks posed by public swimming pools, it's not a guarantee.

When used in the proper amounts needed to maintain a consistent pH, chlorine eliminates all manner of nasty bugs. Chlorinated pool water kills E. coli in less than a minute. Norovirus and Hepatitis A perish after 15 minutes. Giardia lasts for only about 45 minutes in properly chlorinated water.

However, Cryptosporidium, a microscopic parasite that causes a diarrheal disease that can last two to three weeks, is different. Due to a protective coating during part of its life cycle (we'll get to that in a minute), Cryptosporidium, commonly known as Crypto, can survive for up to 10 days in the pool, even one that is properly maintained. According to the Centers for Disease Control and Prevention, in recent years Cryptosporidium has become the leading cause of swimming pool-related outbreaks of diarrheal illness in the United States.

Any time an infected person passes any amount of feces into the water, they're also releasing the parasite. At this point in its life cycle, Crypto is encased in a hardy, thick-walled shell, a form known as an oocyst. When a swimmer somehow swallows or inhales water that contains an oocyst, the parasite enters the body. It then moves through the digestive system and into the cells lining the gut, where it begins to multiply rapidly. Up to 100 million oocysts can be released with a single bowel movement.

It's not just swimming pools that are at risk when used by people infected with Crypto. Water parks, hot tubs and recreational fountains can become contaminated as well. So can lakes and the ocean, although there, with the vastly larger volume of water, the odds of contact are somewhat reduced.

Ironically, the very chemicals used to keep swimming pool water clean can themselves become health hazards. The scent of chlorine, particularly in indoor pools where adequate ventilation can be difficult, is a sign that chloramines, a compound present in chlorinated water, has turned to gas. Chloramines can cause nasal irritation, coughing or wheezing, and can even trigger asthma attacks.

Your best defense is insisting on good pool etiquette.

-- Choose a pool that insists patrons take a pre-swim shower.

-- Make sure that anyone with diarrhea never goes into the pool.

-- If you have young children, take them on frequent bathroom breaks.

-- Remember: Diapers don't belong in swimming pools.

-- Any time you see feces in the water, or when the smell of chlorine becomes strong, tell pool management.

(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. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: What are the risks of a colonoscopy? Based on what I know about the procedure, it seems entirely likely that the colon could be damaged.

Dear Reader: Many people share your concerns, especially if they've never undergone the procedure. The first thing you should know is that colonoscopy is the gold standard in colorectal cancer screening. It can locate and remove polyps that may later increase the risk of colon cancer, and it's critical to the diagnosis of colon cancer in its early stages, when the disease is most curable. Colonoscopy may not be perfect, but studies have found decreased mortality rates among people who have a colonoscopy between the ages 50 and 75. That being said, the colonoscopy is not an entirely benign test.

First, there is the bowel preparation necessary to clear the colon. For some people, the medications to empty the bowels can cause abdominal bloating, pain, nausea, vomiting and alterations of the body's electrolytes.

Many patients who undergo colonoscopies are sedated prior to the procedure, and in people with a previous history of heart disease or severe lung disease, the sedatives can lead to abnormal heart rhythms, a drop in blood pressure and a decrease in the blood oxygen level. These problems are rare, but those with heart and lung disease should be aware of the risks.

After the preparation, the gastroenterologist inserts the scope and adds air to distend the colon, an act called insufflation. Here's where the primary risk comes in: The scope and the insufflation of the colon can indeed lead to perforation (tearing) of the colon. Perforations are more likely to occur after the age of 75; the perforation rate is nearly 6 times higher in that age group than in people under 60. Also, people who are ill from other diseases, such as heart failure, diabetes or cancer, have a greater rate of perforation. So, too, do those with diverticula of the colon (small pouches caused by weakness in the colon wall). Removal of a large polyp also increases the risk of perforation. Large perforations require surgical repair, while small perforations can be managed without surgery. Worldwide, perforation rates from a screening colonoscopy occur once in every 1,428 colonoscopies.

Other risks arise from polyp removal -- standard procedure upon their discovery -- because it can cause bleeding, and this risk increases in patients with a bleeding disorder. To reduce the likelihood of bleeding, all patients having a colonoscopy should stop taking any blood thinners several days beforehand.

As for infections, this complication is very rare and is related to defective equipment or improper cleaning of equipment.

The overall death rate from both screening and diagnostic colonoscopies is about 1 in 35,000, and the rates are continuing to decline.

Although there is a risk from colonoscopies, you should be aware that the risk of colon cancer is much higher -- more than 40 cases per 100,000 people each year in the United States -- and the colonoscopy is essential in the diagnosis of this and other colon diseases. If your doctor says you should have one, I would urge you to follow that advice.

(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. Owing to the volume of mail, personal replies cannot be provided.)

Dear Doctor: Is it true that chemotherapy not only makes you lose your hair, but that it can also change the color and texture of hair that grows back, and even get rid of gray?

Dear Reader: We think your question may be addressing two different types of cancer treatment -- chemotherapy and immunotherapy. And in both cases the answer is yes; each can have an effect on the regrowth of hair.

After decades of use, much is known about the side effects of chemotherapy. These powerful drugs target cells in the body that grow and divide rapidly. However, chemotherapy drugs are indiscriminate. They can't tell the difference between the rapidly dividing cancer cells and the cells that form our hair, eyebrows and eyelashes.

Many cancer patients undergoing chemotherapy experience hair loss. Depending on the type of treatment, it can range from partial, with perceptible thinning of hair on the scalp and body, to complete hair loss, including body hair, eyelashes and eyebrows. When hair begins to grow back, usually three to six months following treatment, it can indeed be different from the hair that was lost.

For some patients, hair that was previously straight before treatment may grow back slightly coarse and quite curly. We've had some of our patients refer to this as "chemo curl." Changes in color, while not common, are also possible. Once the cells that control hair growth and color recover from the effects of chemotherapy and begin to function again, many patients find that their hair reverts to the color and texture it was before treatment.

Your question about cancer therapy returning color to gray hair makes us think you're talking about recent news stories emerging from the field of immunotherapy. Unlike chemotherapy, which directly targets cancer cells, immunotherapy drugs enlist a patient's own immune system in the fight against cancer cells. It's a new and often experimental approach to treatment, and the full range of potential side effects is not yet understood.

Last year, researchers followed 52 lung cancer patients who were each taking one of a trio of immunotherapy drugs in order to monitor any side effects the treatments might produce. They were surprised to see that 14 of the patients had their hair color go from gray to shades that varied from dark brown to black. One of the 14 patients had patches of black interspersed in his gray hair.

Of these 14 patients, 13 responded well to the immunotherapy drugs. This has led researchers to suspect that the change in hair color is linked to -- or is even an indicator of -- how well the cancer drugs are working. The researchers concluded that the sample size is small enough that these changes in hair color can be random flukes, so they plan to continue to investigate.

If you're hoping that immunotherapy can be used to reverse or stave off the onset of gray hair, you'll have to be patient. At this point in time, immunotherapy drugs have very serious side effects, so their use on otherwise healthy people is still a long way off.

(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. Owing to the volume of mail, personal replies cannot be provided.)