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Trees enhance life in a multitude of ways. They combat climate change by reducing greenhouse gases in the atmosphere. They muffle sound pollution and reduce air pollution, drawing in carbon dioxide and releasing oxygen. When rain pours from the skies, trees decrease stormwater runoff, preventing flooding and soil erosion. They also provide valuable habitats to support biodiversity in insects, birds, and other animals, and microorganisms. The list goes on.

Equally important is accumulating evidence that simply spending time around trees and green spaces uplifts our health and mood. Below are a few of the biggest benefits we reap.

Keeping it cool: Trees help prevent heat-related illness

Climate change is causing rising temperatures and more heat waves across the US. These effects are worse for those who live in neighborhoods known as urban heat islands, where asphalt and concrete soak up heat during the day and continue to radiate it at night. Temperatures can reach 7° F hotter than suburban, rural, or simply wealthier and leafier neighborhoods.

Trees and their leafy canopy provide shade that helps to prevent urban heat islands. What does that mean for individuals? It translates to fewer heat-related health illnesses, which strike outdoor workers and younger, older, and medically vulnerable people more often. A study published in The Lancet calculated that increasing tree canopy to 30% coverage in 93 European cities could prevent an estimated four in 10 premature heat-related deaths in adults in those cities.

How trees help children: Better mood, behavior, attention, and more

Spending more time in nature has been linked with better health outcomes like lower blood pressure, better sleep, and improvement in many chronic conditions in adults. These findings are prompting a growing interest in forest therapy, a guided outdoor healing practice that leads to overall improved well-being. But what’s also remarkable are the varied benefits of trees and nature for children.

One study of children 4 to 6 years old found that those who lived close to green space demonstrated less hyperactive behavior and scored more highly on attention and visual memory testing measures compared with children who did not.

Just seeing trees can have mental health benefits. In Michigan, a study of children between the ages of 7 and 9 demonstrated that students who could see trees from their school windows had fewer behavioral problems than those with limited views.

In Finland, researchers modified daycare outdoor playscape environments to mimic the forest undergrowth. These daycares were compared to control standard daycares and nature-oriented daycares where children made daily visits to nearby forests. At the end of 28 days, the children in the daycares with modified forest undergrowth playscapes harbored a healthier microbiome and had improved markers of their immune systems as compared to their counterparts.

How green space helps communities

Having green space in neighborhoods also does a lot to enrich the well-being of communities. A randomized trial in a US city planted and maintained grass and trees in previously vacant lots. Researchers then compared these green spaces to lots that were left alone.

In neighborhoods below the poverty line, there was a reduction in crime for areas with greened lots compared to untouched vacant lots. Meanwhile, residents who lived near lots that were greened reported feeling safer and increased their use of the outside space for relaxing and socializing.

How can you help?

Unfortunately, urban tree canopy cover has been declining over the years. To counter this decline, many towns and nonprofit organizations have programs that provide trees for planting.

A few examples in Massachusetts are Canopy Crew in Cambridge and Speak for the Trees in Boston. (Speak for the Trees also offers helpful information on selecting and caring for trees). Neighborhood Forest provides trees for schools and other youth organizations across the US. Look for a program near you!

Planting trees native to your region will better suit the local conditions, wildlife, and ecosystem. Contact your regional Native Plant Society for more information and guidance. If you are worried about seasonal allergies from tree pollen, many tree organizations or certified arborists can give you guidance on the best native tree selections.

If planting trees is not for you but you are interested in contributing to the mission, consider donating to organizations that support reforestation, like The Canopy Project and the Arbor Day Foundation.

About the Author

Wynne Armand, MD, Contributor

Dr. Wynne Armand is a physician at Massachusetts General Hospital (MGH), where she provides primary care; an assistant professor in medicine at Harvard Medical School; and associate director of the MGH Center for the Environment and … See Full Bio View all posts by Wynne Armand, MD

Not so long ago, a friend texted me from a coffee shop. He said, "I can't believe it. I'm the only one here without a tattoo!" That might not seem surprising: a quick glance around practically anywhere people gather shows that tattoos are widely popular.

Nearly one-third of adults in the US have a tattoo, according to a Pew Research Center survey, including more than half of women ages 18 to 49. These numbers have increased dramatically over the last 20 years: around 21% of US adults in 2012 and 16% of adults in 2003 reported having at least one tattoo.

If you're among them, some recent headlines may have you worried:

              Study Finds That Tattoos Can Increase Your Risk of Lymphoma (OnlyMyHealth)

              Getting a Tattoo Puts You At Higher Risk of Cancer, Claims Study (NDTV)

              Inky waters: Tattoos increase risk of lymphoma by over 20%, study says (Local12.com)

              Shocking study reveals tattoos may increase risk of lymphoma by 20% (Fox News)

What study are they talking about? And how concerned should you be? Let's go through it together. One thing is clear: there's much more to this story than the headlines.

Why are researchers studying a possible link between tattoos and lymphoma?

Lymphoma is a type of cancer that starts in the lymphatic system, a network of vessels and lymph nodes that twines throughout the body. With about 90,000 newly diagnosed cases a year, lymphoma is one of the most common types of cancer.

Risk factors for it include:

• advancing age
• certain infections (such as Epstein-Barr virus, HIV, and hepatitis C)
• exposure to certain chemicals (such as benzene, or possibly pesticides)
• family history of lymphoma
• exposure to radiation (such as nuclear reactor accidents or after radiation therapy)
• having an impaired immune system
• certain immune diseases (such as rheumatoid arthritis, Sjogren's disease, or celiac disease).

Tattoos are not known to be a cause or risk factor for lymphoma. But there are several reasons to wonder if there might be a connection:

• Ink injected under the skin to create a tattoo contains several chemicals classified as carcinogenic (cancer causing).
• Pigment from tattoo ink can be found in enlarged lymph nodes within weeks of getting a tattoo.
• Immune cells in the skin can react to the chemicals in tattoo ink and travel to nearby lymph nodes, triggering a bodywide immune reaction.
• Other triggers of lymphoma, such as pesticides, have a similar effect on immune cells in lymph nodes.

Is there a connection between tattoos and lymphoma?

Any potential connection between tattoos and lymphoma has not been well studied. I could find only two published studies exploring the possibility, and neither found evidence of a compelling link.

The first study compared 737 people with the most common type of lymphoma (called non-Hodgkin's lymphoma) with otherwise similar people who did not have lymphoma. The researchers found no significant difference in the frequency of tattoos between the two groups.

A study published in May 2024 — the one that triggered the scary headlines above — was larger. It compared 1,398 people ages 20 to 60 who had lymphoma with 4,193 people who did not have lymphoma but who were otherwise similar. The study found that

• lymphoma was 21% more common among those with tattoos
• lymphoma risk varied depending on how much time had passed since getting the tattoo:
  • within two years, lymphoma risk was 81% higher
  • between three and 10 years, no definite increased lymphoma risk was detected
  • 11 or more years after getting a tattoo, lymphoma risk was 19%

There was no correlation between the size or number of tattoos and lymphoma risk.

What else should you know about the study?

Importantly, nearly all of the differences in rates of lymphoma between people with and without tattoos were not statistically significant. That means the reported link between lymphoma and tattoos is questionable — and quite possibly observed by chance. In fact, some of the other findings argue against a connection, such as the lack of a link between size or number of tattoos and lymphoma risk.

In addition, if tattoos significantly increase a person's risk of developing lymphoma, we might expect lymphoma rates in the US to be rising along with the popularity of tattoos. Yet that's not the case.

Finally, a study like this one (called an association study) cannot prove that a potential trigger of disease (in this case, tattoos) actually caused the disease (lymphoma). There may be other factors (called confounders) that are more common among people who have tattoos, and those factors might account for the higher lymphoma risk.

Do tattoos come with other health risks?

While complication rates from reputable and appropriately certified tattooists are low, there are health risks associated with tattoos:

• infection, including bacterial skin infections or viral hepatitis
• allergic reactions to the ink
• scarring
• rarely, skin cancer (melanoma and other types of skin cancer).

The bottom line

Despite headlines suggesting a link between tattoos and the risk of lymphoma, there's no convincing evidence it's true. We'll need significantly more research to say much more than that. In the meantime, there are more important health concerns to worry about and much better ways for all of us to reduce cancer risk.

About the Author

Robert H. Shmerling, MD, Senior Faculty Editor, Harvard Health Publishing; Editorial Advisory Board Member, Harvard Health Publishing

Dr. Robert H. Shmerling is the former clinical chief of the division of rheumatology at Beth Israel Deaconess Medical Center (BIDMC), and is a current member of the corresponding faculty in medicine at Harvard Medical School. … See Full Bio View all posts by Robert H. Shmerling, MD

All of us have an internal clock that regulates our circadian rhythms, including when we sleep and when we are awake. And light is the single most important factor that helps establish when we should feel wakeful (generally during the day) and when we should feel sleepy (typically at night).

So, let me ask you a personal question: just how dark is your bedroom? To find out why that matters — and whether sleeping in an eye mask is worthwhile — read on.

How is light related to sleep?

Our circadian system evolved well before the advent of artificial light. As anyone who has been to Times Square can confirm, just a few watts of power can trick the brain into believing that it is daytime at any time of night. So, what’s keeping your bedroom alight?

• A tablet used in bed at night to watch a movie is more than 100 times brighter than being outside when there is a full moon.
• Working on or watching a computer screen at night is about 10 times brighter than standing in a well-lit parking lot.

Light exposure at night affects the natural processes that help prepare the body for sleep. Specifically, your pineal gland produces melatonin in response to darkness. This hormone is integral for the circadian regulation of sleep.

What happens when we are exposed to light at night?

Being exposed to light at night suppresses melatonin production, changing our sleep patterns. Compared to sleeping without a night light, adults who slept next to a night light had shallower sleep and more frequent arousals. Even outdoor artificial light at night, such as street lamps, has been linked with getting less sleep.

But the impact of light at night is not limited to just sleep. It’s also associated with increased risk of developing depressive symptoms, obesity, diabetes, and high blood pressure. Light exposure misaligned with our circadian rhythms — that is, dark during the day and light at night — is one reason scientists believe that shift work puts people at higher risk for serious health problems.

Could sleeping with an eye mask help?

Researchers from Cardiff University in the United Kingdom conducted a series of experiments to see if wearing an eye mask while sleeping at night could improve certain measures of learning and alertness.

Roughly 90 healthy young adults, 18 to 35 years of age, alternated between sleeping while wearing an eye mask or being exposed to light at night. They recorded their sleep patterns in a sleep diary.

In the first part of the study, participants wore an intact eye mask for a week. Then during the next week, they wore an eye mask with a hole exposing each eye so that the mask didn't block the light.

After sleeping with no light exposure (wearing the intact eye mask) and with minimal light exposure (the eye mask with the holes), participants completed three cognitive tasks on days six and seven of each week:

• First was a paired-associate learning task. This helps show how effectively a person can learn new associations. Here the task was learning related word pairs. Participants performed better after wearing an intact eye mask during sleep in the days leading up to the test than after being exposed to light at night.
• Second, the researchers administered a psychomotor vigilance test, which assesses alertness. Blocking light at night also improved reaction times on this task.
• Finally, a motor skill learning test was given, which involved tapping a five-digit sequence in the correct order. For this task, there was no difference in performance whether participants had worn an intact eye mask or been exposed to light at night.

What else did the researchers learn?

No research study is ever perfect, so it is important to take the conclusions above with a grain of salt.

According to sleep diary data, there was no difference in the amount of sleep, nor in their perceptions of sleep quality, regardless of whether people wore an eye mask or not.

Further, in a second experiment with about 30 participants, the researchers tracked sleep objectively using a monitoring device called the Dreem headband. They found no changes to the structure of sleep — for example, how much time participants spent in REM sleep — when wearing an eye mask.

Should I rush out to buy an eye mask before an important meeting or exam?

If you decide to try using an eye mask, you probably don’t need to pay extra for overnight shipping. Instead, follow a chronobiologist’s rule of thumb: “bright days, dark nights.”

• During the daytime, get as much natural daylight as you possibly can: go out to pick up your morning bagel from a local bakery, take a short walk during your afternoon lull at work.
• In the evening, reduce your exposure to electronic devices such as your cell phone, and use the night-dimming modes on these devices. Make sure that you turn off all unnecessary lights. Finally, try to make your bedroom as dark as possible when you go to bed. This could mean turning the alarm clock next to your bed away from you or covering up the light on a humidifier.

Of course, you might decide a well-fitted, comfortable eye mask is a useful addition to your light hygiene toolkit. Most cost $10 to $20, so you may find yourself snoozing better and improving cognitive performance for the price of a few cups of coffee.

About the Author

Eric Zhou, PhD, Contributor

Eric Zhou, PhD, is an assistant professor at Harvard Medical School. His research focuses on how we can better understand and treat sleep disorders in both pediatric and adult populations, including those with chronic illnesses. Dr. … See Full Bio View all posts by Eric Zhou, PhD