Volatile organic compounds (VOCs) are the precursors of secondary particulate and ground-level ozone, which are formed in the atmosphere by reactions of VOCs and oxides of nitrogen (NOx) in the presence of sunlight. Most of the VOCs themselves are harmful to human health. Their photochemical products, namely smog and ozone, have a wide variety of human health effects and cause agricultural crop loss and damage to forests and ecosystems, as discussed in Chapters 1 and 5 of this book.