Why You Need to Exercise Outdoors
I rarely frequent the gym. Sometimes I go once or twice a week for a weight training/circuits class, but I haven’t been into that lately. I also swim 1-2 times per week (which in Alaska is indoor exercise…no outdoor pools here), but otherwise a majority of my exercise occurs outside. In the winter I also bike on the trainer 1-2 times per week, but you better bet I do my longer bike rides outside.
Also, please note that if the only thing that keeps you physically active or motivated to live a healthy lifestyle is your gym membership, then that is much better than sitting on the couch eating popcorn and watching Real Housewives.
the problem with indoor gyms
Recent studies have shown high levels of carcinogens in the air of gyms as well as a variety of harmful bacteria lurking on the surfaces of most fitness equipment. One study found high levels of carbon dioxide, formaldehyde, and other volatile organic compounds (VOCs) and other particle pollution was present in multiple indoor fitness centers. 
We breathe out carbon dioxide, so CO2 levels rise when there are a lot of people enclosed in an area, especially if the ventilation isn’t great. This problem gets worse when we are working harder, such as during exercise. This particular study noted that the highest levels of CO2 were in an interior room that was used for indoor spin classes and cycling. Even though these levels of CO2 were not high enough to be considered toxic, they aren’t completely harmless either. Too much carbon dioxide can make it harder to breath and it can make you feel lightheaded or dizzy. This is probably not how you want to feel while you are working out.
So the more people who are running, riding bikes, lifting weights, and just generally breathing heavily that you try to cram into an indoor space (think about the gym at 5:30pm during the first week of January), the worse the air quality gets. Because of this, I think that you are better off exercising in a home gym or in a small group fitness situation if you do need to exercise inside or need the motivation and structure of the gym.
And then, there’s mold…
Mold is a common problem in moist climates. It’s common in gyms because they are full of sweaty people breathing hard, swimming pools, showers, and saunas that get used all day long every day. That creates more than enough humid air for mold to go gangbusters. So, you go into the gym that’s full of sweaty, humid, moldy air and you start working out and breathing hard, you’re probably breathing in some spores from the mold, which can cause undesirable respiratory and other health issues. 
Are you grossed out yet?
The other issue is something called “particulate matter.” Particulate matter is made up of solids and liquids like dust and other things that get knocked loose into the air. These particles are small enough to pass through your nose and into your lungs, especially when you are breathing hard. Particulate matter isn’t just an issue in the gym, but it’s certainly worse there because you have a lot of people doing things that disrupt dust and other particles and send stuff into the air (things like plopping rubber mats on the floor, banging dumbbells together, and breathing hard).
Over 25% of the gyms in the study exceeded the acceptable limit for these types of particles. 
Also, over 80% of the gyms exceeded the acceptable level of VOCs as well. VOC levels are higher in gyms that have new equipment or in spaces that have been recently cleaned (due to the chemicals used during cleaning).  Exposure to VOCs at higher than acceptable levels can cause skin irritation, neurotoxic and hepatoxic effects, and some of them are carcinogens. 
And last, but not least, we have all those synthetic fragrances. Air fresheners, perfumes, colognes, and deodorants that are all over the place (not just in the gym, though they are particularly concentrated there). Depending on what it’s made out of, synthetic fragrance can be an endocrine disruptor and contribute to asthma. 
These are not all issues that are unique to gyms, however they are of an increased concern in a gym as it’s assumed that you are going there to improve your health, not make it worse. The air quality issues are more of a concern as well because you are typically breathing more deeply when you are exercising, putting you at an increased risk.
The benefits of Outdoor exercise
The list of benefits from being outside is impressive. Something as simple as walking in the woods, hiking, or just spending time amongst the trees exposes you to phytochemicals that are released by plants which helps to decrease cortisol, depression symptoms and anger. 
One study showed that being exposed to bacteria and “other environmental variables” as a child has a positive impact on longevity and health outcomes. In cultures that have become too sanitary (specifically in relation to not being outside around dirt, animals, and other “microbial ecology”) there is an increase in depression, anxiety, and immune-related diseases, and a decrease in overall health. 
Another study found that certain environmental elements such as the sound of running water, the smell of wood, and the scenery in the forest decreases stress as measured by cortisol levels, pulse, and blood pressure.  Several other studies have found that sunlight can decrease depression and that spending as little as five minutes in nature can improve mood, self-esteem, and motivation. 
These studies highlight how while exercise is good for our bodies, it can also benefit our brain, especially if you make sure to get outside as much as possible. Also, most people don’t stick with regular gym attendance beyond the first year, but people who exercise outside tend to do so more consistently (partly due to outdoor exercise being more restorative than indoor exercise). 
how to exercise outside
After all of this you might be ready to get out of the gym and move your workouts outside, but how do you do that? It can be intimidating to change your perspective about exercise as something that happens inside a gym to using nature as your gym.
1. Turn Your Commute Into A Workout
Yes, biking or running or skiing to work takes a little bit of extra time, but it’s time well spent. Rather than sitting on your butt in the car for 20 minutes, turn that time into a 60 minute cardio workout. You could also kill two birds with one stone and bike/run/ski to the gym to do your strength workout. You might also be surprised at the time you free up even though it takes longer to get where you are going.
Hiking and trail running are two of my very favorite outdoor activities. Probably because speed is not usually the goal. In the summer time when I get together with friends, it’s usually for a hike. Sometimes we hike with coffee in our hands, but the point is to be outside moving our bodies. Hiking can be anything from a lovely stroll down a trail to something more difficult (add a backpack or weight vest and try to go faster). You can typically find trail maps for state parks nearby online, or there is the app AllTrails which gives tons of info on hiking trails almost everywhere.
3. Make Your Own Gym At A Park
Use park benches to do dips, split squats, and box jumps. Do pull-ups on monkey bars or rings. Exercises like burpees, squats, lunges, or push-ups can be done anywhere. You can also do sprints on the trails or yoga in the grass. Some places even have parks that have circuits built into them. It’s often just a matter of looking to see what’s nearby.
4. Use Nature As Your Gym
You can use heavy objects found in nature, such as rocks or large tree branches, as weights or obstacles to climb (or jump) over. Planting a garden and doing yardwork requires a lot of healthy movement patterns (squats, lunges, fireman carry, etc.). This article from Breaking Muscle has an entire outdoor circuit that you can do using only rocks and trees.
5. Get Out On The Water
Kayaking, stand up paddle boarding, and swimming are all great workouts. Some of these may not be possible during certain seasons depending on where you live, but they are great options when available.
How to exercise Outside in the winter
Living in Alaska, I absolutely understand the challenges of outdoor exercise during the winter. It’s cold, dark, and can be full of snow or ice, making it feel like even more of a chore to get your body moving.
1. Dress Appropriately
Invest in some good layers. This will make or break your outdoor exercise experience. My layers of choice are wool because they breathe well and still keep me warm even when they get soggy from sweat. I prefer Icebreaker for my base layers. It lasts a long time with moderate to heavy use, so while it’s an investment up front, you’ll have these pieces for a long time.
I also recommend dressing for about 15 degrees warmer than what the thermometer says. This will make for a cold start to your exercise, but once you get moving and warmed up, you’ll be glad you didn’t put that extra layer on.
2. Be Visible
Short days during the winter can make it hard to get your workout in during daylight hours. So, if you are exercising outside in the dark, make sure you’re visible. I prefer a reflective vest since I can throw it on over the top no matter how many layers I’m wearing, and it doesn’t get in my way or restrict my movement. A headlamp can also be nice depending on where you are running (if I’m running on neighborhood streets I wear it for visibility, if I’m running on the trails I have it with me but don’t turn it on since it’s easier to see without it).
My dogs also have visibility gear. I have a lighted LED vest for one and a reflective vest for the other. Since they are low to the ground (and one is black and hard to see in the dark), it’s nice to have them be visible to cars as well.
3. Watch Your Step
The footing can be tricky on your run or walk outside during the winter months. You could invest in some spiked shoes, or try these strap on grippers to keep you from slipping and sliding through your workout.
If you are into winter biking, make sure you get some studded tires.
Outdoor Exercise & air pollution
I realize that outdoor exercise is not exempt from exposure to air pollution, which can be just as bad as indoor air pollution in some areas, though in most places the average indoor air quality is worse than the outdoor air quality.  Regardless of whether the city you live in has poor air quality (Los Angeles, CA has some of the worst air quality and North and South Dakota the best), you’ve probably done a workout on a busy road at some point in your life.
Workout out in or near air pollution outside is the same as working out in a gym. Smaller amounts of air pollutants can damage lungs when you are breathing hard, they easily pass through your nasal hairs which is your body’s first line of defense when it comes to allergens and pollutants in the air. 
One study done in 2005 had healthy subjects exercise for 30 minutes on stationary bikes in a lab where they had piped in diesel exhaust fumes at levels that mimic a busy highway during rush hour. Researchers found that the blood vessels of these subjects were less able to distribute oxygen to their muscles. They also found that these people had much lower levels of “tissue plasminogen activator” which functions as a natural blood clot buster. The conclusion of this study was that working out along polluted roads can increase your risk of heart attack or stroke.  
Newer research does offer some hope to those who have no choice but to exercise in more polluted urban areas. In this study two groups of mice were used. Both were exposed to regular doses of exhaust fumes for five weeks, but the first group did not exercise and the second group did. Researchers found a large spike in the inflammation of the lungs and in free radicals in the mice who did not exercise however the mice who exercised in those same conditions appeared to have undergone adaptations that allowed their bodies to counteract the harmful effects of the pollution.  These findings caused the researchers to conclude that long-term aerobic exercise might offer some protective effects against the negative impact of pollution. This is potentially due to the ability of the body to use and produce antioxidants.
Another similar study conducted in humans had two groups of cyclists one of which cycled at various intensities while exposed to diesel exhaust and another who performed similar activity in clean air. The study found that in almost all of the areas measured, high-intensity exercise while exposed to the exhaust did not differ from those who exercised in fresh air. They concluded that their findings do not support the advice to decrease exercise intensity on high pollution days but recognize that this is based on data from healthy males and doesn’t address all the potential health effects of air pollution exposure. 
While additional studies need to be performed to determine if the anti-inflammatory effects of exercise are truly protective against pollution, hopefully these few studies can help to alleviate some of your concerns. It seems that making sure you get out and exercise is more important than not exercising at all if you live in an area that has significant air pollution.
So, don’t stop exercising…even if it’s in a gym or an area that has pollution. If you have the option to exercise outside in fresh, clean, air…do that as much as possible!