Dear Doctor: I've heard that cellphones increase the risk of brain tumors -- and also that they don't. Which is true?

Dear Reader: First, let's look at brain tumors in general. They are classified as either benign or malignant, meaning cancerous. The most common type of benign brain tumor is a meningioma, which arises from the meninges, the outer lining of the brain and spinal cord. The most common type of malignant brain tumor is a glioma, which arises from glial cells that line the individual nerve cells.

Ionizing radiation from nuclear accidents or explosions, radiation therapy, or even too-frequent dental X-rays can increase the risk of brain tumors. This type of radiation can lead to mutations within the DNA, increasing the risk of tumor cells.

Cellphones and cordless phones, on the other hand, emit radiofrequency energy, a form of non-ionizing radiation. This type of radiation has little potential to cause DNA damage. From that perspective, because cellphones produce non-ionizing radiation, their risk of cancer should be pretty low. Now let's check out the data.

A 2015 Swedish study in the Journal Oncology Reports looked at meningiomas diagnosed between 1997 and 2009. People with meningiomas filled out questionnaires asking about their use of cellphones, home cordless phones and analog phones (phones attached to a wire). The authors also asked whether the patients had placed the phone toward one ear or the other, or used both ears equally.

The authors found no statistical difference between mobile phone use and cordless phone use when compared to analog phones. However, those with the greatest amount of cordless and cellphone use had a higher rate of meningiomas compared to those with the lowest amount of cordless and cellphone use.

The interesting aspect of this is that those who frequently used analog phones also had more meningiomas than those who used analog phones less frequently. So this research carries the possibility of recall bias, in which someone with a disease may overly represent a possible risk factor.

A 2017 analysis of many studies, published in the journal Neurological Sciences, found no overall association between cellphone use and brain tumors. Rather, the authors found that many studies on the topic were methodologically flawed.

The authors did note that the higher-quality studies showed a correlation between cellphone use and brain tumors. The authors also concluded that in the higher-quality studies, long-term use of mobile phones was linked to an increase in the risk of brain tumors. Long-term use was defined as greater than 10 years, or more than 1,640 hours.

Another 2017 analysis, published in the International Journal of Occupational Medicine and Environmental Health, reached a similar conclusion.

It's difficult to make judgments based on this data. There does appear to be some evidence that more than 10 years of cellphone use may increase the risk of brain tumors. Yet the potential for recall bias means the data aren't irrefutable. Nonetheless, I would recommend being conscious of the amount of time you spend with a cellphone or a cordless phone against your head. Luckily, we can now communicate through a Bluetooth device, reducing whatever risk there may be.

One encouraging note? Rates of both benign and malignant brain tumors decreased in the United States between 2009 and 2013.

(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: My husband, who plays in a weekend soccer league, is troubled by some new reports he has read that suggest that soccer might be as dangerous as football. Can you explain the studies?

Dear Reader: Your husband may be referring to a pair of studies published in February that examined the potential risks of heading the ball, a common move in soccer. One study, conducted at a British university, found signs of brain damage in a group of professional soccer players, all of whom had decades-long careers. The other study, published by researchers at Albert Einstein College of Medicine in New York, focused on amateur players. The study found an increased risk of concussion both from heading the ball and from accidental contact during a game.

But before you insist that your husband turns in his cleats, let's take a closer look at the data.

The British study examined 14 retired soccer players with dementia. The men, skilled at heading the ball, played for an average of 26 years each. When the brain tissue of six of the players was examined after their death, researchers found signs of trauma.

In four of the six brains, this included chronic traumatic encephalopathy (CTE), a progressive degenerative disease also found in professional boxers, football players and hockey players. Although the small sample size of the British study kept researchers from drawing conclusions, they agreed the results reveal the need for large-scale studies.

In the American study, researchers followed 222 amateur soccer players, both men and women. Players reported how often they headed the ball, as well as any accidental blows to the head during play. They also rated the severity of symptoms like headache, dizziness, altered vision, weakness or confusion, which can indicate concussion.

The researchers concluded that while heading the ball increased a player's risk of concussive impact, accidental contact, like an elbow to the head or colliding with another player, was even more common.

Research into sports-related concussions has previously focused on football and hockey, where bruising impacts are part of the game. Now, scientists are expanding their inquiries into soccer.

Soccer is the world's most popular team sport. It is estimated that amateur players head the ball up to 12 times per game. Add in practice drills, and this can add up to 2,000 headers over 20 years. The question now is whether the effect of all those headers on the delicate structures of the brain can be cumulative.

For the authors of the American study, the answer was yes. They recommend that players be aware of any concussion symptoms, even absent a conclusive concussion diagnosis. Should symptoms appear, players must avoid another collision in the following weeks, when the risk of another concussion spikes significantly.

The truth is, all physical activity carries both risks and benefits. One thing we discuss with our patients regarding soccer is the risk of knee injury, given the high lateral impact forces.

It sounds as though your husband is staying abreast of the latest research, which is wise. He can use it to decide how -- and even whether -- he decides to play soccer in the future.

(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: A recent study found that air pollution increases the risk of Type 2 diabetes. How can that be? Air pollution can't possibly raise blood sugar levels.

Dear Reader: I can understand your disbelief. My first thought when I saw this study was that it was correlative, not causative. In other words, air quality is simply worse in cities, which are more likely to have large numbers of people with lower socioeconomic status, who, in turn, tend to have greater rates of obesity and diabetes. Then I looked more closely.

Air pollutants assessed in the studies include nitrogen dioxide and particulate matter between 0.1 to 2.5 microns in size (PM 2.5). Early epidemiologic studies showed a correlation between areas of greater pollution and diabetes, but did not control for socioeconomic status. Then came a 2010 study that used census data from three counties in the United States, and assessed both rates of diabetes and Environmental Protection Agency data regarding PM 2.5 pollution.

The authors took into account such socioeconomic variables as median income, high school completion, male sex and ethnicity -- all markers for diabetes risk. The authors found a 16 to 20 percent increase in the prevalence of diabetes in the areas that had the highest amounts of PM 2.5 versus those areas with the lowest amounts, even with all other factors being equal.

Then came this year's study published in the journal Diabetes that included 314 obese Latino children in Los Angeles. The authors analyzed where the children lived; the amount of pollution to which they were exposed, measured by both nitrogen dioxide and PM 2.5; and their parents' socioeconomic data. The children were followed for 3.4 years, during which time they underwent tests of blood sugar, two-hour glucose tolerance, insulin and insulin response to glucose. The authors found that the markers for future diabetes were much greater in those exposed to greater amounts of pollutants. This was independent of socioeconomic status, and even independent of the level of obesity.

The cause isn't completely clear yet. Studies in rodents exposed to pollutants have shown increased inflammation within fat cells, accumulation of cholesterol in the liver and decreased ability of muscles to use sugar. This leads to both metabolic dysfunction and obesity. Further, pollutants can lead to systemic inflammation that in turn leads to insulin resistance, and thus the inability to bring sugar into the cells of the body, leaving it to sit in the bloodstream.

What is clear is that there does appear to be an association between pollution and diabetes, but more studies are needed to evaluate the degree of the association. Regardless, pollution is obviously unhealthy and needs to be controlled within society. The measurements of pollution provided in these studies come from the Environmental Protection Agency, whose existence is to safeguard the health of current and future societies.

(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.)