Dear Doctor: Because I have fair skin and live in an area with a lot of sun, I try to do my outdoor activities early in the day (before 9 a.m.) or late in the afternoon (after 4 p.m.). Do I still need to wear sunscreen?

Dear Reader: In a word, yes. If your goal is to protect your skin from sun damage, which includes sunburn, wrinkles and a range of cancers, our advice is to wear sunscreen during the daylight hours. In fact, research now shows that even a light tan is a sign of sun damage.

This dawn-to-dusk timing for sunscreen application may seem counterintuitive; after all, sunlight looks and feels markedly weaker during the early morning and late afternoon than at midday. While it's true that the sun's rays are strongest between 10 a.m. and 4 p.m., the fact is that from the moment it appears above the horizon to the instant it vanishes from sight, our closest star is sending a full dose of skin-damaging ultraviolet radiation our way.

How can that be?

Sun damage is caused by two types of radiation -- ultraviolet A and B, better known as UVA and UVB. These are part of a broad spectrum of light, which includes the light we can see. But because ultraviolet rays are shorter than rays of visible light, they can't be seen with the naked eye. However, what you can't see definitely can hurt you.

The so-called "sunburn ray" is UVB. It has a slightly shorter wavelength than UVA, and is not as concentrated in the early morning and late afternoon. The highest amounts of UVB radiation reach us between 10 a.m. and 4 p.m. in the months of April through October. And while UVB accounts for only 5 to 10 percent of all UV radiation that reaches us, it plays a key part in the onset of skin cancer.

By contrast, UVA maintains the same intensity all day, which makes dawn-to-dusk sunscreen use important. UVA is responsible for signs of aging, like wrinkles and dark spots. It also damages certain cells in the basal layer of the epidermis, which is where most skin cancers form. In the past, UVA was absolved of a cancer connection. Now, however, researchers believe that it plays a role in cellular changes that lead to cancer.

The other thing to know is that UVA can penetrate clouds and glass. When you're riding in a car, sitting near a window or spending time outdoors in poor weather, you're still in the path of UVA rays. Reflective surfaces, such as water, snow, ice and glass, refract up to 80 percent of the UVA and UVB rays that hit them. That means you're getting close to a double dose of the sun's harmful rays. And don't forget that the sun's rays become more potent at higher altitudes.

Our final word of advice is to be sure to use sunscreen rated for both UVA and UVB radiation. It will be clearly marked on the label. Use it generously and reapply according to product instructions. Your skin will thank you.

(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: Every now and then I read about a death related to "flesh-eating bacteria" that was contracted in water of some sort. The most recent one was in the floodwaters of Hurricane Harvey, near Houston. How does this happen? Is there anything we can do to stay safe?

Dear Reader: When we talk about flesh-eating bacteria, we're really talking about necrotizing fasciitis, a serious skin infection that can be caused by one of several different types of bacteria. The bacterium enters the body through a break in the skin -- anything from an insect bite to a scrape, scratch or open wound -- and then rapidly spreads through the body's soft tissue.

Contrary to their name, these bacteria don't actually eat the flesh. However, they do kill it. The word "necrotizing" means "causing the death of tissues." The word "fasciitis" refers to the fascia, which is the connective tissue that surrounds muscles, fat, blood vessels and nerves.

As for the bacteria themselves, group A strep is the most common cause. Other possibilities include E. coli, staph (aka Staphylococcus aureus), Klebsiella, Clostridium and Aeromonas hydrophila. In the case of Harvey's floodwaters, which included seawater, the marine bacterium Vibrio is also a possible culprit.

Although several cases of necrotizing fasciitis came to light due to contact with the contaminated floodwaters of Hurricane Harvey, the condition is rare. About 700 cases are diagnosed per year in the United States. In fact, many people exposed to the bacteria that cause it are not affected. According to the Centers for Disease Control and Prevention, those who do contract it often have weakened immune systems due to conditions like diabetes, cancer, chronic lung, heart or kidney disease, or autoimmune diseases like lupus.

There is no surefire way to prevent necrotizing fasciitis. That means the best defense is using good wound care techniques. For example:

-- Never delay first aid for a wound, even if it's minor or not infected. A blister, scrape or any break in the skin should be cleaned, disinfected and covered.

-- If you do have an open wound, or one that is draining, always keep it covered with clean and dry bandages until it is healed.

-- If you have an open wound or a skin infection, steer clear of enclosed systems like hot tubs, swimming pools or whirlpools. You should also avoid contact with natural bodies of water, like rivers, lakes, ponds and oceans.

Vigilance is key. Necrotizing fasciitis moves swiftly. Unlike a typical infection, which develops over the course of days, the symptoms of necrotizing fasciitis begin within hours.

Many patients report experiencing pain that is more severe than expected for the size of their wound. Ulcers, blisters or black spots may appear. The skin may feel quite warm, with areas of swelling that have a reddish or purple cast. Later, flulike symptoms including chills, fever, extreme fatigue and vomiting may appear.

Successful treatment depends on the earliest-possible intervention. Even then, broad-spectrum antibiotics and surgical debridement may not stop the infection. In some cases, amputation becomes the only option to save the patient's life.

If after performing first aid you are concerned about a wound, or simply feel better being hypervigilant, use a pen to outline the borders of redness when any infection appears. If the infection grows beyond those borders in the course of hours, seek out medical help immediately.

(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: In the last few years I've noticed that after making an appointment to see a doctor, I'm often seen by a physician's assistant or nurse practitioner instead. What's the difference? When should I insist on being seen by a physician?

Dear Reader: You're correct -- nurse practitioners and physician's assistants are now playing larger roles in preventive health care for many Americans. While they can't replace physicians, what they can do is take on a significant range of duties in primary care. This frees up doctors to manage the big picture of a patient's health care and to spend more time with difficult, complex cases.

Let's start with some definitions.

The difference between a physician, a physician's assistant and a nurse practitioner is in their schooling. Each is a nationally certified and state-licensed medical professional who has successfully completed four years of college as an undergraduate.

Aspiring physicians go from college to four years of medical school, followed by an additional three to seven years of specialized training as residents. Many then undergo additional training in fellowships, which can last from one to three years.

For physician's assistants, or PAs, college is followed by three years of medical training in an accredited program. More than 2,000 hours are spent in clinical rotations that include family medicine, internal medicine, general surgery, emergency medicine, pediatrics, and obstetrics and gynecology. In order to enter practice, PAs must pass a national certifying exam and qualify for a state license. All PAs are required to work with a supervising physician.

A nurse practitioner, or NP, is a registered nurse whose advanced training allows her or him to perform an expanded range of duties. These include physical exams, diagnosing and treating chronic diseases like diabetes and arthritis, ordering and interpreting diagnostic tests, and prescribing medications. An NP can perform procedures like stitching a wound, setting a break and performing a skin biopsy. Their focus is preventive medicine, wellness and education. Depending on the state, NPs may work independently of a physician's supervision.

Both PAs and NPs have a role in surgery, but to what extent depends on laws and regulations, which vary greatly from state to state.

Like many of our colleagues, we see a role for PAs and NPs in preventive care, such as well-woman and well-baby exams, geriatric assessments, and in urgent care cases that are not complex or severe. Here at UCLA, many hematologists and oncologists work alongside an NP. The NPs help care for complex cancer patients, coordinating imaging, therapy and routine follow-up visits.

By handling certain parts of a medical visit, like collecting data, compiling a medical history and assessing general health, PAs and NPs free up physicians' time so they can focus on more complex issues.

This leads us to the second part of your question, which is when to ask to see a physician. We suggest that you make it a practice to ask whether a doctor is available during your appointment. That way, if an exam or test reveals anything troubling or abnormal, if you want a second opinion about something the PA or NP has said or if you have questions that you prefer to have answered by your doctor, then you know he or she is available to you.

(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.)