The mechanism by which plants clean contaminated air is twofold:
Plants absorb office pollutants into their leaves and transmit the toxins to their roots, where they are transformed into a source of food for the plant.
Plants emit water vapors that create a pumping action to pull dirty air down around the roots, where it is once again converted into food for the plant.

Wolverton has found plants are especially beneficial in office buildings in which Sick Building Syndrome is common, suggesting that everyone should have a plant on their desk, within what he calls the "personal breathing zone." This is an area of six to eight cubic feet where you spend most of your working day. Jon Naar, author of "Design for A Livable Planet: How You Can Help Clean Up the Environment" (Harper & Row, 1990), suggests 15 to 20 plants are enough to significantly improve the air quality in a 150square meter area.

A two-year study conducted by Tove Fjeld, a professor at the Agricultural University, Oslo, found the following reductions in ailments in an office after plants were introduced:

Ailment Reduction
Fatigue 20%
Headache 45%
Sore/dry throat 30%
Cough 40%
Dry facial skin 25%

Humidity; Another Important Factor in Health
Humidity levels play an important role in employee health. When humidity levels are too low, individuals are more likely to develop a cold or catch the flu; when levels are too high, vulnerability to disease and illness increases

Most buildings do not have systems to maintain humidity within desirable ranges. Those with humidification systems often have problems with humidifiers that become contaminated by microorganisms. When the relative humidity of interior air is too low, workers develop colds and virus infections more frequently.

Similarly, high relative humidity in buildings causes numerous problems. The condensation of windows and exterior walls in winter can result in expensive structural damage. Molds and mildews grow when relative humidity exceeds 75 percent, and dust mites multiply faster in environments with a higher relative humidity. Again, workers exposed to unhealthy conditions become more vulnerable to disease and illness.

Relative humidity inside buildings should be maintained to prevent damage or harm caused by high or low levels of moisture. Buildings are routinely designed to remove humidity by venting interior air to the outside. Without the exchange of air, interior relative humidity would rise to saturation because there are many sources of moisture in most buildings: People release moisture through their skin and as they breathe, and moisture may be emitted from cooking and washing.

Plants control not only the toxin levels in the air, but also the humidity. Interior plants maintain the relative humidity in offices to within the approved human comfort range. A study conducted by Dr Virginia Lohr at Washington State University determined that when plants were placed in offices, the relative humidity increased significantly and actually stabilized at the recommended range of 30 to 60 percent. In the absence of plants, the relative humidity in offices was slightly below the recommended range for human comfort levels.

The research recorded the relative humidity of office space in a building with a central, forced-air system in the presence and absence of plants. Measurements were taken during four consecutive winter months. Once each week, plants were added or removed as required. Humidity and temperature were recorded every six hours. A variety of plant species were used. Air exchange rates were estimated to average one to two air changes per hour.

Plants contribute to interior humidity by adding moisture to the air through transpiration and, secondarily, through evaporation from the growing media. The relative humidity stabilizes because plants naturally reduce their levels of transpiration when relative humidity is high and increases the rate of transpiration when lower relative humidity is present. The study also concluded that plants did not contribute excessive amounts of moisture to any of the interior spaces studied.

Conclusion
All of the research carried out on the subject concludes that the simple addition of interior plants significantly reduces the level of airborne contaminants and controls humidity, these being two most likely causes of sick building syndrome.

See our "health benefits" page for more information on the benefits of houseplants