The microbiome is our own inner forest of beneficial and harmful microbes. Bacteria, yeasts and other types of microscopic organisms live within our gastrointestinal tract and on our other body surfaces. Beneficial bacteria help protect us from less helpful bacteria and yeast and other fungi. The moisture rich air of a forest has been found to have a variety of beneficial nutrients and may even provide some beneficial bacteria. 
Moisture rich air also can provide a positive lift for the mood from negatively charged ions. The smell of ozone in the air may be noticeable after a thunder storm or on the beach of a large lake or ocean. In excess ozone can cause headaches but smaller amounts may have a positive affect on the mood. 
Other research has found that simply looking at images of nature helped reduce stress levels for the study participants. Walking through urban areas has not been found to have as much stress reduction benefits as a similar amount of time spent walking through a more natural setting. 
Some types of indoor plants may provide additional benefits besides stress reduction or lift the mood. Some species of common indoor houseplants also help clean the air of chemicals commonly found in modern environments which may have negative effects on health. Volatile chemicals are released over time from plastics such as vinyl. New carpets and vinyl flooring may release larger amounts of volatile chemicals when they are first installed and continue to release smaller amounts over the years. Formaldehyde is one of the volatile chemicals which has been associated with health risks for adults and children and neurological risks for a developing fetus.
A study by NASA regarding what types of plants might help remove the negative chemicals found in the air of enclosed buildings led to a list of 50 houseplants that are effective at cleaning the air of negative chemicals. The complete list is available in a book and the top ten are listed online. The book goes into more detail about which plants may be better for removing which types of chemicals from the air of enclosed spaces – such as NASA spaceship. 
Volatile chemicals that can make an indoor environment more hazardous to breathe and which led to the term “sick building syndrome” include “formaldehyde, Volatile Organic Compounds (benzene and trichloroethylene or TCE), airborne biological pollutants, carbon monoxide and nitrogen oxides, pesticides and disinfectants (phenols), and radon.” 
The book by Dr. B.C. Wolverton, the scientist who did the research for NASA, is called “How to Grow Fresh Air – 50 Houseplants that Purify Your Home or Office.” It includes more detail about which plants may be better for cleaning which types of chemicals from the air and includes basic guidance on the type of care and ease of care that each species of plant might require to keep it alive. 
Caution is needed to not over-water houseplants or to leave them in a poorly drained pot as it may lead to mold. Some types of mold can have negative health risks but the type common to a houseplant are more likely to simply smell musty which can have a negative impact on the air of an enclosed space.
Houseplants which require misting such as ferns may also be increasing the amount negatively charged ions of water in the air which may help promote positive moods for people in the room. Small electronic model waterfalls also can increase the amount of negatively charged water ions in the air. Some types of air purifiers include an ozone generator however they aren’t recommended by the EPA as too much ozone can have negative health consequences. 
An early indicator of too much ozone might be a headache. The ozone generating units may be useful for freshening the air of an enclosed space when there are no pets or humans in the room. Levels of ozone necessary to kill “biological organisms” (such as mold spores perhaps) would also be irritating or harmful for humans to breath for very long however but can make a difference when used in an empty enclosed space at a higher setting, , and once the ozone generating machine is turned off the increased levels of ozone return to normal after a few hours in my personal experience and the room can be safely used again. Roughly — the ozone chemically clumps with other particles in the air, deodorizing and causing the heavy clusters to fall from the air and can be removed by vacuuming or mopping the floor.
I have used one of the ozone air cleaners in my home and it could cause obvious increases in ozone smell if turned up to full generating potential and that could lead to a headache for me. At lower power levels the ozone generation did seem to help freshen the air and it did seem to have a positive effect on my mood. * This information is provided not as a recommendation rather as a caution. The ozone generating air cleaners can be useful but can also be dangerous.
Electricity exists in many forms — in laptops and in a misty morning breeze.
For more time later – oxidative stress can be caused by lack of oxygen or too much, or emotional and other physical reasons  and it may be involved in how ozone could have positive effects at low levels and negative effects at increased levels. 
Disclaimer: Opinions are my own and the information is provided for educational purposes within the guidelines of fair use. While I am a Registered Dietitian this information is not intended to provide individual health guidance. Please see a health professional for individual health care purposes.
- Kelly Tatera, “Scientists Reveal Why “Forest Bathing” or Going to the Beach Boosts Our Well-Being,” Feb. 3, 2016 
- Jared Green, “Research Shows Nature Helps with Stress,” 9/8/2011 
- Greg Seaman, “The Top 10 Plants for Removing Indoor Toxins,” May 13, 2009 
- B. C. Wolverton, “How to Grow Fresh Air: 50 House Plants that Purify Your Home or Office Paperback,” April 1, 1997 
- U.S. Environmental Protection Agency, Indoor Air Quality (IAQ), “Ozone Generators that are Sold as Air Cleaners” 
- Samima Selim, Oxidative Stress and Psychological Disorders, Curr Neuropharmacol. 2014 Mar; 12(2): 140–147. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3964745/ 
- Masru Sagai, Velio Bocci, Mechanisms of Action Involved in Ozone Therapy: Is healing induced via a mild oxidative stress?, Med Gas Res. 2011; 1: 29. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3298518/ 
- Byoung-Suk Kweon, Roger S. Ulrich, Verrick D. Walker and Louis G. Tassinary, Anger and Stress: The Role of Landscape Posters in an Office Setting, Environment and Behavior 2008 40: 355 https://www.researchgate.net/profile/Louis_Tassinary/publication/258132363_Anger_and_Stress_The_Role_of_Landscape_Posters_in_an_Office_Setting/links/0a85e5304d07b6d045000000.pdf