Nitrogen dioxide, or NO2 is a highly reactive gas. NO2 forms primarily as a result of burning fuel, such as emissions from cars and trucks. Factories, power plants and other large-scale operations also put off NO2. As a serious air pollutant, NO2 harms the human body over time.
So what does nitrogen dioxide have to do with carpets? NO2 can cause fading in carpets, generally starting at the tips of the yarn. The carpet dye for the blue and violet colorations is especially sensitive to fading by nitrogen dioxide. When this fading or color change takes place in the carpet due to NO2, there is no way to reverse the process.
Interestingly, carpets made of wool have lots of air pockets within the fibers, which allows it to effectively absorb a number of indoor air pollutants, including NO2, sulphur dioxide and formaldehyde.