Your afternoon workout started off great. Despite temperatures creeping toward the upper 80s, your legs felt fresh, and you began running through Washington Park at a brisk pace. But now, you’ve barely covered a mile, and you just feel… off. Your chest tightens, and your lungs seem unsatisfied no matter how hard you suck air. Frustrated, you slow to a walk and begin to cough.

While it’s possible you overestimated your fitness level, those symptoms are indicative of another problem: air pollution. Ground-level ozone, particulates seen and unseen, and a variety of other gases and compounds lurk in the air and harm your body, especially your lungs and sinuses, when you breathe them in, says Dr. Scott Joy, an internal medicine practitioner with Englewood Primary Care at Swedish Medical Center. “Pollutants make airways sensitive and swollen,” he says. “People can start to feel poorly, particularly with a cough or shortness of breath.”

Such symptoms present a problem for the fit, outdoorsy set Colorado and its capital city often attract. After all, it’s tough to enjoy your bike ride along the Cherry Creek Trail or soccer game at City Park when you can’t catch your breath. And on top of the state’s bad reputation with ozone—according to the American Lung Association’s 2021 State of the Air report, the Denver-Aurora area placed eighth on the dreaded list of the 25 Cities Most Polluted by Ozone, while Fort Collins came in 17th—longer and more intense wildfire seasons are spewing higher levels of lung-irritating smoke and debris across Colorado.

It’s a lot to consider when all you’re trying to do is squeeze in a quick jog. Luckily, Joy has been living and practicing in Colorado since 2012, and as a fan of cycling and hiking, he’s familiar with the decision-making athletes must navigate on hot, smoggy days. While you should consult your doctor about your specific concerns and needs, Joy can offer some education and guidelines to help you decide whether it’s safe to take your sweat-fest outdoors.

Understand Short-Term Symptoms and Long-Term Effects

While ozone, gases, and particulates interact with your body in slightly different ways, they typically have one thing in common: They cause inflammation. When your airways swell, the passageway through which oxygen reaches your lungs grows smaller, which is why you feel short of breath and may experience wheezing, coughing, and a tight feeling in your chest.

Pollution in the air you inhale through your nose messes with your sinuses, too. Pollutants collect on the mucus lining your upper respiratory system, causing swelling, congestion, and post-nasal drip. You might even notice some pain and swelling around your eyes, cheeks, nose, and forehead.

Those annoying-in-the-moment symptoms can have long-term impacts on your health if you’re chronically exposed to air pollution, according to Joy. “Your body doesn’t like to be exposed to noxious stimuli for a long period of time,” he says. “Airways that are really elastic become stiffer and develop scar tissue if that assault continues.” That scar tissue, in turn, can prevent medications from working properly.

New research also suggests that continuous pollution exposure can harm the cardiovascular system, too. A study published in the peer-reviewed European Heart Journal in April 2021 examined how physical activity and air pollution levels impacted the cardiovascular health of nearly a million adults ages 20 to 39. The young adults, who all lived in metropolitan areas of South Korea and started the study without any signs of cardiovascular disease, were examined from 2009 to 2010 and again from 2011 to 2012.

A decrease in physical activity for subjects facing exposure to low-to-moderate levels of air pollution was associated with a higher risk of cardiovascular disease, which makes sense given what we know about the health benefits of exercise. But those who did the highest level of physical activity while being exposed to high levels of particulate matter also faced a higher risk of cardiovascular disease, including coronary heart disease and stroke.

The researchers in part blame fine particulate matter for harming the cardiovascular systems of the ardent exercisers in the study. The tiny molecules easily enter the bloodstream, which then deposits them into the organs causing constriction of blood vessels (including the ones on the heart’s surface), increases in blood pressure, and the clumping together of platelets in the blood, a step on the way to blood clot formation. Inflammation and oxidative stress also occurs, both of which are risk factors for future cardiovascular disease.

What You Can Do to Protect Yourself

If all that left you feeling a little freaked out… well, same. But don’t panic—Joy has some pretty straightforward advice for those concerned about balancing the health benefits of exercise with the major downsides of breathing in polluted air.

First of all, be thoughtful about when you work out. Hot temperatures and direct sunlight exacerbate ozone conditions, so Joy recommends fitting in your workout in the morning or evening, when the sun is less of a factor. “If you’re looking for optimal times, it’s usually before 10 a.m. and after 4 p.m.,” he says. Getting outside after a rainstorm is also a good time—the moisture drags pollen and particles down to the ground, so they’re no longer floating around ready to invade your airways.

Hydration is also key, Joy says. Drinking enough water maintains your airway’s mucus lining, which, while gross-sounding, helps protect your tissues from certain pollutants. Plus, when those tissues dry out, they can crack, making them more likely to get inflamed (remember, inflammation leads to scar tissue). You should also prioritize sleep to help reduce inflammation.

Always bring any necessary medication, like your prescribed rescue inhaler, when you exercise, and if you’re hiking outside Colorado’s urban areas, don’t let the seemingly clean mountain air fool you into leaving such supplies behind. “Particularly with the wildfires, you could be out on a clear hiking trail and the wind changes, and all of a sudden you get a lot of smoke,” Joy says. “If your airways get irritated you don’t have your rescue inhaler, that’s not a good afternoon.”

His other tip for hikers? Always bring a LifeStraw so you have a way to get water, even when you’ve emptied your Nalgene.

Ultimately, it’s about listening to your body. “Recently, I was going for a bike ride, but it was 95 degrees out,” Joy says. “So I decided to take a shorter route. If you feel like your body is telling you not to push hard, there’s a reason for that.”

Along those lines, if you’re starting to notice new symptoms like shortness of breath or coughing while you’re working out, go to your doctor. “We get a lot of people who have undiagnosed asthma and then reach their threshold,” Joy says. Hot weather and increased air pollution may be the factors that demarcate that threshold, so don’t ignore symptoms. “You might be a patient who would benefit from some maintenance therapy,” he says. “We’ve had patients who start taking medication and are surprised at how much better they feel, even if they didn’t notice a problem before.”

Your doctor can help you create an asthma action plan to manage and treat your symptoms, but even if you don’t have asthma, it doesn’t hurt to do some research and have a plan of your own. Colorado Department of Health and Environment releases a detailed Colorado Air Quality Summary each day that lets the public know the air conditions of their respective region. The page uses the Air Quality Index (AQI), the Environmental Protection Agency’s scale for reporting the condition of the air, which runs from zero to 500. Most people can safely exercise outdoors when levels are under 150, though sensitive individuals should use their best judgment anywhere between 50 and 150. Once levels exceed 150, though, even members of the general population may experience side effects. At level 300, no one should be exercising outdoors.

The department also posts a Colorado Smoke Outlook each day to keep residents updated on how wildfires are impacting air quality. Both the outlook and the AQI can help you decide if the benefits of an outdoor workout outweigh the potential air pollution risks—or if it might be better to stick to the treadmill.

(Read More: How To Protect Your Skin From Denver’s Air Pollution

Angela Ufheil
Angela Ufheil
Angela Ufheil is a Denver-based journalist and 5280's former digital senior associate editor.