Dear Doctor: I'm 84 and live in a big city. I try to walk as many places as I can, but I've read that air pollution may shorten the lifespan of seniors. Should I stay inside instead?

Dear Reader: As we enter our later years, we become increasingly vulnerable to health issues like chronic illness and infection, and to the negative effects of environmental factors like air pollution. At the same time, our immune systems are changing. As they become less robust, we lose the ability to respond to and recover from adverse conditions.

You are correct that studies have found a direct correlation between the degree of outdoor air pollution in a particular location and the lifespans of the individuals who live there. While air pollution affects people of all ages -- it's connected to at least 3.7 million premature deaths per year worldwide -- it's particularly hard on the elderly and the very young. In older individuals, it has been shown to exacerbate the effects of chronic disease and respiratory tract infections.

Outdoor air pollution is generated by emissions from traffic, factories and other industries, power generation, wood-burning fireplaces, wildfires, wind-blown dust and particulates, which are minute solid particles and liquid droplets suspended in the air. Ongoing exposure to outdoor air pollution has been tied to the increased incidence of conditions like asthma, chronic obstructive pulmonary disease (COPD), chronic bronchitis and emphysema. Researchers now have evidence that long-term exposure to outdoor air pollution can have an adverse effect on both heart and lung function in otherwise healthy individuals.

That's quite a bit of somewhat dire information, so we're happy to follow it up with steps you can take to reduce the risks of both indoor and outdoor air pollution, no matter where you live.

Let's start with your concerns about walking outdoors. We believe that the physical and mental health benefits of physical activity are significant enough that you should make every effort to stick to your fitness routine. If you have internet access, you can check on the air quality in your city each day at airnow.gov. This information is also provided in many television and radio weathercasts. When conditions are right and you do go walking, steer clear of the busier streets and avenues. Find quiet streets with minimal traffic for your walks. And remember to always wear sunscreen. Using an umbrella or parasol for shade and added coolness can also be a good idea.

When air quality is less than optimal, your instincts are correct -- you should limit your time outdoors. And though we've focused on air quality, it's important to be aware of the dangers that heat poses to elderly individuals. On days with high temperatures, take your walk in the early morning. When it's already too warm in the morning, stay inside near a fan or an air conditioner. And if your home is too warm, please take advantage of your local senior citizen center, or the cooling stations that most cities make available to residents during heat waves.

(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 exactly is the connection between birth control pills and breast cancer? Is the evidence credible? If so, why do so many women still take them -- and so many doctors prescribe them?

Dear Reader: The reason women take them, and doctors prescribe them, is quite simple: Hormonal contraceptives (birth control pills) are an effective means to prevent pregnancy. They are widely available, used by an estimated 140 million women worldwide. This equates to 13 percent of women ages 15 to 49.

The hormones in these pills are either estrogen and progesterone or progesterone alone. Concern about the possible connection to breast cancer centers on the fact that many types of cancer are stimulated by estrogen and progesterone. So hormone-containing pills could theoretically increase the risk for breast cancer.

Older studies assessing this risk have shown either a very mild increase in risk or no increase. A 1996 article in the journal Lancet combined data from 54 studies in 25 countries assessing the risk of breast cancer among women who had used hormonal contraception of various types, including pills, injectables and IUDs. Overall, the study found a small increased risk -- 7 percent -- of breast cancer among women who had ever used hormonal birth control. Among current users, it found a 24 percent increased risk. In women who had stopped taking hormonal birth control for either one to four years or five to nine years, it found a 16 percent and 7 percent increased risk, respectively. No increased risk was found among women who had stopped taking hormonal birth control more than 10 years prior.

Then came a 2017 study in the New England Journal of Medicine of 1.8 million women, ages 15 to 49, in Denmark. The women were followed for an average of about 11 years and, over that timeframe, 11,517 were diagnosed with breast cancer. Those who had ever used hormonal birth control (oral or implantable) had a 20 percent increased risk of breast cancer when compared to women who had never used it. Those who had used it for less than one year had a 9 percent increased risk over never-users, and those who had used hormones for more than 10 years had a 38 percent increased risk.

The risk did not vary based on the type of oral contraceptives, nor was there a difference in risk between oral contraceptives and hormone-containing implantable birth control. When the authors analyzed the risk posed specifically by oral contraceptives, they found an increased risk of 24 percent compared to non-users of any hormonal contraceptive.

A natural question is: What if a woman has an increased risk of breast cancer already? A 2013 study combined data from eight studies of women who were carriers of a BRCA gene mutation that increases the risk for breast cancer. It found a 21 percent increased risk of breast cancer among oral contraceptive users, meaning the risk was no different for women with a higher risk of breast cancer.

Here's another way to look at it: The number of breast cancers caused by hormonal contraceptives is one extra breast cancer case per year (above what already would be expected) for every 7,690 women.

In short, although oral contraceptives may slightly increase the risk of breast cancer, the risk isn't enough to recommend that women stop using a reliable, largely safe method of birth control.

(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: How does the flu virus spread? I've always thought it was mostly from coughing, which contaminates the air and the surfaces we touch. But now I've heard that people who are sick actually breathe the virus out. Should I be wearing a mask to protect myself?

Dear Reader: One of the more challenging points to get across regarding the flu has been just how contagious a sufferer is, and how early into a bout of the illness that contagion begins. Now comes research that makes understanding (and acting on) this information all the more crucial.

You're correct that the assumption has been that it's by coughing that we pump aerosolized droplets of moisture, which contain the flu virus, into the air and onto the surfaces around us. We've known for some time that these droplets can hang in the air for up to several hours and can live on hard surfaces for several days. But the results of a recently published study by scientists at the University of Maryland's School of Public Health show that by simply breathing, a person who is sick with the flu is exhaling the virus.

The researchers examined the mechanics of how 142 university students with confirmed cases of influenza sent the virus into the air around them. The participants in the study sat in front of a device that could measure various sizes of the aerosolized droplets they dispersed while coughing, sneezing, talking or breathing. In 218 sessions that lasted 30 minutes each, it was revealed that close to half of the airborne droplets that contained the flu virus were collected in the absence of coughing. Since the participants rarely sneezed, the takeaway was that people with an active infection are dispersing the virus into the air around them when they breathe.

An earlier study from the University of Hong Kong concluded that half of flu infections within households are passed along via aerosolized droplets. The study also looked into your question of whether wearing a surgical mask would prevent infection. The answer echoes the findings of several similar studies -- basically, no. Though masks may slightly reduce risk by screening out the largest droplets, they are not effective against the fine aerosolized mist from the breath. According to the Centers for Disease Control and Prevention, the people who really should be wearing facial masks are individuals who are ill.

It's important to note that the University of Maryland study did not tackle the specific question of how the flu is transmitted. Still, the findings do suggest that, even in the absence of coughing, people with the flu can send the virus into the environment around them.

All of which leads to advice that physicians have been giving -- and which patients have been ignoring -- for generations. And for emphasis, we're putting it into a paragraph of its own:

Stay home when you're sick.

Not only do you really and truly need to rest in order to give your body the best shot at recovering from influenza, but venturing forth while you're actively ill puts everyone around you at risk.

(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.)