Although people tend to think that being indoors means being safe and protected, World Health Organization (WHO) studies show that indoor air is often 2-5 times more polluted than outdoor air. 1
When you realize that people spend approximately 90% of their time indoors , it’s even more important to ensure that the air you breathe indoors is clean air.
1 Actual values were indoor/outdoor ratios of 1.62–6.37 for the summer months and 2.05–10.99 for the winter months. Text and Figure from: T. Salthammer Angewandte Chemie Int. Ed. 52, (2013), 3320; Original data from: B. Wang et al. / Atmospheric Environment 41 (2007)2851–2861.