Dear Doctor: I always reach for the highest SPF when I'm buying sunscreen, especially for the kids. But my mom says what matters most is the type of UV rays the sunscreen is blocking. Why is that? What does SPF actually measure?

Dear Reader: There's a lot of confusion when it comes to sunscreen. We've had patients admit that when they get overwhelmed by the labels and the choices while shopping for sunscreen, they sometimes give up and decide to go without. But sunscreen can help prevent skin cancer. In fact, it is estimated that exposure to the sun's UV rays is responsible for up to 90 percent of non-melanoma skin cancers and at least 65 percent of melanomas. And while it's not quite in the same life-or-death category, UV rays are also responsible for signs of premature aging like wrinkles, brown spots and sagging skin.

Let's start with the basics. Sunlight is the portion of the sun's radiation that reaches us at the Earth's surface. It's made up of three basic components -- the visible light that we see, infrared rays, which deliver heat, and ultraviolet rays. Although we neither see nor feel ultraviolet radiation, it still manages to make its presence known. On the plus side, it triggers our bodies to manufacture vitamin D, an essential micronutrient. But prolonged exposure to the sun's ultraviolet spectrum results in skin damage.

The longer wavelengths of ultraviolet light, known as UVA, are the so-called tanning rays and are known to age the skin. UVB rays, which cause sunburn, play a major role in skin cancer. Although you may hear the axiom regarding UV rays that A is for aging and B is for burn, the most recent research suggests that both types of UV rays play a role in each process. All of which brings us to your question about SPF factors and sunscreen.

SPF stands for sun protection factor. While the ascending numbers of the SPF scale seem to imply an increasing strength or concentration of the sunscreen product, what they actually represent is the length of time that you can spend in the sun before starting to burn. Let's say it takes five minutes in the sun without protection before your skin begins to redden. A sunscreen with an SPF of 10 means that when you apply it, you will have 10 times that amount of time -- 50 minutes -- before you begin to burn. A lotion with an SPF of 30 gives you 150 minutes. Bottom line -- the "factor" in SPF is the number by which you multiply your minutes-to-a-burn time.

By using sunscreen, you've applied a chemical barrier that helps keep UV radiation from reaching the skin. The trick here is to be sure you use a full-spectrum (sometimes known as a broad-spectrum) sunscreen. This means the product blocks both the UVA and UVB rays, which is essential for the best protection. And while this seems like a no-brainer, you really do need to follow the instructions on the product in order to get full protection. Apply at least 15 minutes before heading outside, use at least an ounce or two for adequate coverage, and reapply at least every two hours.

(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: I was recently diagnosed with COPD and prescribed two inhalers and two nebulizers, only to quickly develop severe cramps in my feet. I noticed, however, that when I take Mucinex (guaifenesin), the foot cramps disappeared completely. Is it safe to continue to take it?

Dear Reader: Guaifenesin was first approved by the Food and Drug Administration in 1952 as an expectorant. In this capacity, the medication works to thin out mucus secretions in the sinuses and chest and thus facilitate its movement out of the body. Robitussin and Mucinex are brand names of guaifenesin. Similar to other medications, guaifenesin does more than what it was intended for.

Guaifenesin's use as a muscle relaxant may be related to its ability to block the NMDA and glutamate receptors. The blocking of these receptors has also shown anti-seizure activity in mice given high doses of guaifenesin.

A 2017 study looked at 77 subjects between the ages of 18 and 25 who came in to the doctor's office for upper back, neck and shoulder pain. Those individuals were assigned different doses of guaifenesin to take -- 600 milligrams, 1,200 milligrams or placebo twice a day for seven days. During the study period, the participants could not use NSAIDs nor Tylenol. Twice a day over the seven-day period the subjects recorded by questionnaire the intensity of the neck and upper back spasm, stiffness, tension and pain.

There was an improvement of muscle spasm seen each day with both guaifenesin and placebo. But the group that received 1,200 milligrams twice a day of guaifenesin reported a 25 percent greater decrease in muscle spasm compared to the placebo group and a 16 percent greater reduction compared to the 600-milligram group. However, this level of muscle spasm relief was not statistically significant. Still, the 1,200-milligram group did show significant reductions in pain, tension and discomfort.

The greatest benefits with the use of guaifenesin were seen on the fourth day of treatment, but by the seventh day the level of symptoms was not much different than the placebo.

One problem with the study is its small size, so a larger study would be needed to prove the efficacy of guaifenesin in relieving muscular pain and spasm.

The muscle spasms that you are having may be related to a long-acting bronchodilator you are using for the COPD. The guaifenesin appears to help relieve your symptoms, but as you asked, is it safe? Guaifenesin does have side effects for some. Incidents of headache, dizziness, nausea and drowsiness are a little greater than placebo and seen at high doses of the drug. Guaifenesin is often used in patients with COPD to help with mucus plugging, but it is not recommended for this in the long term.

I think it would be safe for you to continue the Mucinex as long as you don't notice side effects from it. I would recommend taking the lowest dose possible to relieve the foot cramps. However, I would also make sure your doctor knows you are taking it and get his or her opinion about using this medication in the long term.

(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: I'm 47 years old and was very happy to have three more years before my doctor started bugging me about a colonoscopy. But my husband says there are new guidelines, and I'm now actually two years late! Why the changes?

Dear Reader: Your husband is correct -- the American Cancer Society recently updated its screening guidelines for colorectal cancer. Rather than wait until age 50, it now recommends people at average risk for the disease should begin regular screenings at age 45. The organization cites the recent rise of colorectal cancer among young people as the impetus for the change. As of yet, neither the United States Preventive Services Task Force nor the American College of Gastroenterology has adopted the new guidelines.

According to a study published last year in the Journal of the National Cancer Institute, the proportion of rectal cancer diagnosed in adults younger than the age of 55 doubled from 1990 to 2013. That means nearly one-third of all diagnoses of rectal cancer now occur among people 55 and younger. Meanwhile, rates of rectal cancer for adults over the age of 55 are continuing their 40-year decline.

Researchers looking into this demographic shift are questioning whether the same lifestyle factors that appear to have a hand in the ongoing obesity epidemic are at work when it comes to the spike of colorectal cancers among younger adults. That is, a sedentary lifestyle, diets that are high in fat and low in fiber, an increase in the consumption of sugar and of highly processed foods, and the weight gain that accompanies these behaviors.

Although colorectal cancer is the fourth most-common cancer diagnosed among adults in the U.S., it is the second-leading cause of cancer death. That's frustrating because colon and rectal cancers are highly curable when caught early. The five-year relative survival rate for people with stage 1 rectal cancer is about 87 percent, according to statistics compiled by the American Cancer Society. For stage 1 colon cancer, that number jumps to 92 percent. Once colorectal cancers spread to other parts of the body, however, they become significantly harder to treat. Metastatic, or stage 4, colon cancers have a five-year relative survival rate of 11 percent. For stage 4 rectal cancers, it's 12 percent.

Catching these cancers early requires regular screening. That's because, in their early stages, colorectal cancers tend to be asymptomatic. Most begin as a growth in the inner surface of the colon or rectum. The majority of these growths, known as polyps, are not malignant. However, certain types of polyps, known as adenomas, have a higher risk of becoming cancers. And that's where the colonoscopy becomes important. Using a flexible tube with a tiny camera, a doctor can scan the interior of the colon to identify potential abnormalities. He or she may also collect tissue samples for biopsy and remove polyps. Stool-based lab tests and special CT scans are also effective.

We know that a colonoscopy can be daunting or even frightening, but the truth is it can save your life. The American College of Surgeons estimates that with regular screening, between 76 and 90 percent of colorectal cancers can be prevented.

(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.)