Ozone is one of the highly reactive gases, which is photo chemically active. It is composed of three atoms of oxygen (O3) and its role depends on its location in the atmosphere. This gas plays a different role in the lowest two layers of the atmosphere, known as the stratosphere and troposphere. In the stratosphere, above the tropopause about 90 percent of the ozone protects life on earth from the sun's dangerous ultraviolet radiation. In contrast, the layer surrounding the earth's surface is the troposphere where the presence of ozone is harmful for the environment as well as for human health. In a polluted urban atmosphere ozone acts as a more powerful photochemical oxidant than other oxidants such as Nitrogen dioxide (NO2), Hydrogen peroxide (H2O2), and Peroxy Nitrogen (PAN). Moreover, ozone in the troposphere is designated as a greenhouse gas due to its absorption in the infrared, visible, and ultraviolet spectral region. Enhancement of ozone in the middle and upper troposphere could have significant climatic consequences. In addition, enhanced ozone levels in the boundary layer of polluted regions have adverse effects on human health and crop yields. The phototoxic nature of tropospheric ozone in relation to photochemical air pollution is discussed in this write-up. Photo chemically produced ozone may affect crop yield, plant growth, and the respiratory system and lung function of humans. Furthermore, it may also damage the building materials. Formation of tropospheric ozone Tropospheric ozone is not directly emitted into the troposphere; rather its presence depends on stratospheric intrusion and complex photochemical reactions. It is considered as a secondary or transformation pollutant rather than a primary pollutant. Primary pollutants that are involved in the formation of secondary pollutants are often referred to as precursors. The formation of tropospheric ozone depends on some precursors such as oxides of nitrogen (NOx), Volatile Organic Compounds (VOCs), and carbon monoxide (CO). Aromatic and olefin hydrocarbons contribute significantly to ozone formation. These precursors act in the presence of sunlight to produce ozone. Since, these reactions are stimulated by sunlight and temperature, the peak ozone levels typically occur in the warmer times of the year during daytime. Due to the increase of anthropogenic emissions in the atmosphere, the growth of carbon and nitrogen compounds are rising dramatically. As a result, the enhanced level of tropospheric ozone has become an issue of concern in terms of photochemical air pollution. Tropospheric ozone was first measured over 100 years ago. Reports mentioned that the average daily maximum of tropospheric ozone in North America was approximately 0.019 ppm, and in Europe 0.017-0.23 ppm. But now this amount exceeds 0.2-0.3 ppm in some cities the world in peak pollution periods. Impacts Impacts of ozone on humans and the environment vary with the emission patterns, meteorological transport and chemical and physical processes. The effects associated with levels of ozone have been monitored in many areas of the world. Effects on human health It is hardly surprising that oxidant like ozone can be damaging to health. Ozone exposure experiments on human health began in the mid 1960s. That exposure data provide a foundation for the interpretation of epidemiological studies and indicate what concentrations are likely to impact on the normal population. It is the potentially serious effects on human health that have caused many countries to adopt air quality standards. Numerous studies indicate that there is an adverse effect on health associated with short-term, prolonged or sub chronic, and chronic exposure to ozone. There has also been growing concern about long-term exposure to elevated ozone levels -- may be cause of irreversible chronic lung injury. Photochemical oxidant can damage respiratory tissues through inhalation. Ozone has been linked to tissue decay, promotion of scar tissue formation, and cell damage by oxidation. Ozone can impair an athlete's performance, create attack that is more frequent for individuals with asthma, cause eye irritation, chest pain, coughing, nausea, headache and chest congestion and discomfort. It can worsen heart disease, bronchitis, and emphysema. On Vegetation Research and analysis indicate that the impacts of ozone on vegetation have also been observed in several regions in the world. Ozone may damage forests and crops. This slows down photosynthesis and plant growth. Ozone can lead to plant tissue injury and reduction in growth and productivity because of its phytotoxic nature. If a sufficient amount reaches sensitive cellular sites within the leaf, ozone exerts a phytotoxic effects. However, ozone injury will not occur if the rate of ozone uptake is slow enough, to allow plants to respond to ozone by defensive reactions, such as avoidance by stomatal closure, detoxification of ozone by chemical reaction, adjustment by alteration of metabolic pathways, and repair of injured tissue. However, these factors depend on the intensity of ozone exposure. On crop yield Ozone may reduce the intended use or value of the plant species, plant communities, or ecosystem. The impact of ozone on crop yield was identified through foliar injury symptoms in the 1960s. Injury symptoms and crop yield are usually not directly proportional because of the importance of variation in allocation processes and metabolic factors in determining plant yield. For instance, much more loss of yield can occur with little foliar injury; on the other hand, foliar injury can be much greater than yield loss. This means the injury -- yield loss relationship may depend on the ozone exposure. As an example, for corn, foliar injury occurred at lower ozone concentration than yield reduction; but as the ozone concentration increased, yield is reduced to a greater extent than the increasing foliar injury. Numerous studies indicate that ambient oxidants reduced the yield and quality of citrus, grape, tobacco, cotton, and potato. The impacts of ambient ozone of oxidants are comparatively much higher than other oxidants. However, studies confirmed that ambient ozone levels are sufficient to reduce crop yields. Higher losses depend on several factors such as ozone concentrations, environmental conditions, and crops that are more susceptible to ozone. Table shows typical ozone injury symptoms of some crops. In fact, ozone can change the integrity of the cells through entering inside a leaf. If the cells collapse and die, then symptoms occur on the leaf surface. Conclusion Intrusion from stratospheric ozone and photochemical production are the two main sources of ozone in the troposphere. The photochemical formation of ozone depends on some precursors as discussed earlier -- VOCs, NOx, and CO. Due to world expansion in agriculture, transportation, and industry, huge amounts of these precursors are emitted into the troposphere. The concentrations of ozone have risen from pre-industrial times to the present because of increased emissions from anthropogenic sources. Ozone, like other pollutants, does not stay in the source region, rather it can transport throughout the global atmosphere. As the fossil fuel emissions are mainly responsible for the photochemical production of ozone, it is therefore important to control the emissions from anthropogenic sources because the global atmosphere is common to all countries and all lives. Khorsheda Yasmeen is UNO, Dhamrai.