Lurking threat

Nascent, nasty and lethal, ozone has risen to dangerous levels in Delhi, and is increasing in various cities -- damaging human health and affecting agriculture

By Anumita Roychowdhury
Last Updated: Sunday 07 June 2015

Lurking threat

-- (Credit: Anand Singh Rawat) going by the limited data that the Central Pollution Control Board ( cpcb ) is willing to part with -- based on monitoring of ozone at a single site in Delhi -- there is no cause for alarm. But according to a report published in September this year by another governmental agency, the Central Road Research Institute ( crri ), ozone had already crossed the danger mark in 1993. cpcb 's claim flies in the face of logic and contradicts its own findings on the magnitude of air pollution in the Capital.

cpcb has been monitoring ozone at the ito crossing for the past eight months. But cpcb officials remain tightlipped about their findings. All that they are willing to say is that ozone levels in the city have been "hovering around 40-45 parts per billion by volume (ppbv), much lower than the World Health Organisation limit of 50-60 ppbv". The cpcb attitude is a slight improvement over its previous stand. In an interview with a researcher from the Centre for Science and Environment in November 1996, cpcb chairperson D K Biswas had altogether dismissed the issue, saying that ozone is "not a problem". While it now recognises the need to monitor ozone levels in the city, it has yet to set up a comprehensive system.

Moreover, cpcb 's claim is in sharp contrast to the evidence presented by the crri . The crri study is based on tests conducted at seven locations in Delhi in the winter of 1993 -- Parliament Street, Daryaganj, Paharganj, Ashram chowk, Karol Bagh, Maharani Bagh and Vasantkunj. Average levels of ozone at five sites over eight daytime hours exceeded the who mean standard of 50-60 ppbv by 10 to 40 per cent (see graph: Ozone rising ).

Even more alarming, the peak levels observed in areas with heavy traffic and congestion like Karol Bagh, Daryaganj, and Parliament Street were as high as 119.0 ppbv, 125.8 ppbv, 123.9 ppbv, respectively. In most cases, the maximum concentration of ozone exceeded the upper who limit by two times.

Scientists point out that even short-term exposure to high levels of ozone can be deadly -- talking about mean or average levels does not help understand the true impact of ozone on health. Studies in other countries have shown that exposure to high ambient ozone over a short period can adversely affect the lungs. It causes inflammation of the airways (bronchus and bronchioles) and leads to typical respiratory problems -- shortness of breath, wheezing and chest pain (see box: Erroneous ozone ). Keeping this in mind, the who has set two standards for ozone concentration. The first, an average measured over eight hours, is 50-60 ppbv. The second, for peak concentrations over one hour, is 76-100 ppbv.

The scope of the crri study was restricted to monitoring ozone for a few months in Delhi and, therefore, did not indicate the seasonal trend in ozone formation. Since the objective of the study was to see the effect of atmospheric inversion on the concentration of pollutants during winter, ozone was monitored along with other pollutants. But Anil Singh of crri emphasises that the trend in ozone formation during summer should be investigated separately: bright sunlight catalyses photochemical reactions in the atmosphere, aiding the formation of ozone.

Scientists had, in fact, sounded the alarm some six to seven years ago. Studies conducted by the School of Environmental Studies, Jawaharlal Nehru University ( jnu ) and the Tata Energy Research Institute ( teri ) in Delhi had shown that peak levels of ozone in the city had touched the 128 ppbv mark in 1989-90 and 114 ppbv in 1990-91, crossing the who safety standard (based on an hourly average) of 76-100 ppbv.

Urban genesis and impact...
Ozone is not emitted directly into the atmosphere. High atmospheric concentrations of ozone are a result of a complex set of reactions in the atmosphere which involves emissions of nitrogen oxides and certain reactive hydrocarbons. Most of the primary pollutants are generated by motor vehicles and industries in cities and towns. Scientists point at the dramatic rise in motor vehicles in Indian cities as primarily responsible for emission of ozone-forming gases like no x and reactive hydrocarbons.

crri scientists had observed a close correlation between peak traffic flow and peak ozone levels in their study. Interestingly, the study indicated that ozone concentrations in winter peaked before noon and in the evening -- closely following the peak traffic trend. Though ozone levels peak at noon in a normal diurnal pattern, the actual pattern showed discrepancies in winter. According to the scientists, this could be due to slow dispersion and trapping of ozone owing to inversion of temperature in the lower atmosphere during evenings and at night. Higher wind speeds at noon could also help in dispersion of ozone.

According to C K Varshney, professor at the School of Environmental Sciences, jnu , ozone formation accelerates during summer due to higher solar radiation. The same factor is responsible for higher ozone levels at noon time, when the sun is brightest. Seasonal variations were noted between 1989 and 1991 in Delhi by Varshney and his team. The maximum concentration was in June 1990, with the monthly mean between March and July ranging from 76 to 81 microgramme per cubic metre (one microgramme is one millionth of a gramme). The mean decreased between September and January to 55-59 microgramme/cu m, but increased between March and July 1991 to 83.45-88.94 microgramme/cu m. Between September 1991 and January 1992 it ranged from 61.85 to 64.11 microgramme/cu m.

Sonia Thimmiah of teri , who has put together these findings in a study, says that ozone at the ground level in Delhi has often far exceeded the who limit (see table: All across the country ). Both crri and jnu scientists feel that ozone is bound to rise in Delhi with increase in emissions of no x , carbon monoxide, methane and other hydrocarbons. This will be facilitated by climatic and weather conditions such as high temperature, intense sunshine, and stable wind.

All across the country
Ozone levels in select cities in India
City Year of monitoring Ozone levels Status with respect to standards



9-128 parts of billion by volume(ppbv) (hourly concentration) Upper level exceeds World Health Organisation(WHO) standard as well as the one used by the US National Crop Loss Assement Network(NCLAN)


15-114 ppbv
Ahmedabad 1991 9-55ppbv(15 minute average)

Exceeds NCLAN standard



29-57ppbv(monthly average based daily mean)

Exceeds NCLAN standard



10-76ppbv(two-hour average) Close to WHO standard


1-34ppbv(hourly mean) Dose not exceed any standard
Source: Sonia Thimmiah, 1996, ‘Air pollution in India with respect to deleterious impact on agriculture,’ Tata Energy Research Institute, New Delhi
...And the rural aftermath
Though emissions from motor vehicles and industries in urban areas are responsible for formation of ozone, the hinterlands are equally vulnerable. Alarming levels of ozone have been noted in rural areas within an 80-km radius of cities by scientists from the Imperial College of Science, Technology and Medicine ( icstm ), London, who, along with scientists from jnu and the Banaras Hindu University have recently investigated the damaging effect of ozone on crops.

Interestingly, some of the ozone generated in cities is mopped up when it combines with nitric acid and other gases in the atmosphere. But the ozone that drifts to rural areas does not get scavenged and remains in the atmosphere for a longer period. As a result, these rural hinterlands are turning noxious. The level of ozone recorded by Varshney and team in Delhi in 1992, for instance, was higher at sites away from the heart of the city. Ozone levels at these sites exceeded the who standard by 20 per cent, while sites in the city violated the standard by 4-9 per cent.

A significant reduction in crop yield has been reported from areas close to cities with high ozone concentrations in the atmosphere. Data supporting this was presented by scientists at a national workshop on the impact of air pollution on agriculture in India held in the second week of June 1997. The workshop, organised by the World Wide Fund for Nature ( wwf ) in association with Imperial College Centre for Environmental Technology, uk, and the icstm , presented the preliminary findings of a project on the impact and cost of air pollution on agriculture in developing countries.

Damage due to ozone in rural areas is estimated to be as high as 40 per cent in case of sensitive crops in Punjab, Haryana, Uttar Pradesh, Gujarat and Maharashtra. A team of scientists led by Nigel Bell, professor at icstm , conducted field experiments on four crops (wheat, rice, cotton and groundnut) at sites close to Mumbai, Ahmedabad, Pune and Lucknow. Their preliminary findings show large local impacts of ozone. The yield loss reported by them is as high as 40 per cent at sites close to Mumbai, Ahmedabad, Pune and Lucknow (see table: Cereal killer ).

"There has been no monitoring of ozone in rural India. The predictions made in our risk assessment were based on data from Pakistan which were extrapolated to India. Using city population size as a surrogate for no x emissions, we calculated the levels of ozone." The scientists said that the threshold for significant yield reduction (at seven-hour mean seasonal concentrations) is lower for sensitive cereals like wheat than for less sensitive crops like rice. The threshold for wheat is 50 parts per billion (ppb), while that for rice is 70 ppb.

"Though mean average concentration of ozone for a season is a workable indicator, it is the peak concentration which is dangerous and we need to keep track of that," says Bell. The scientists have now worked out a new standard for the critical peak level of ozone for plants to estimate damages. The new standard has been developed under the auspices of the un Economic Commission for Europe. The standard says that the level of ozone absorbed by a plant over a growing season of three months should not exceed 3,000 ppb in a daylight hour (over the baseline limit of 40 ppb). Agricultural scientists in Europe feel that this standard is more reliable compared to the one calculated on the dose-response relationship, since it takes into account damaging peak concentrations of ozone.

Vulnerable varieties...
The impact of ozone on agriculture in India was considered by scientists after conclusive evidence was presented by a study in Lahore, Pakistan. The study showed a significant effect on crop yield when the six-hour mean concentration of ozone varied between 40 ppb and 60-70 ppb. Concentration of ozone was generally higher during the rice season than the wheat season (see box: The Lahore experiment ).

Ozone can injure cells and tissues or the whole plant. It penetrates through the stomata (minute apertures in leaves that help in respiration and transpiration), reacts with organic molecules and forms secondary oxidants that damage cell membranes. According to M R Ashmore of icstm , high concentrations of ozone can cause cell collapse, leading to visible injury to the plant and reduction of photosynthetic processes. Plants use carbon dioxide ( co 2 ) for photosynthesis. Exposed to ozone for a long period, plants cannot fix and assimilate co 2 adequately, leading to increased loss of respiratory co 2 . This accelerates ageing and reduces yield in crops. Ozone-induced changes in plants can also alter their sensitivity and ability to react to stress.

Tropical cultivars and plant varieties are more vulnerable to ozone compared to crops grown in the us and Europe. While indicative figures from India show that the loss can be as high as 40 per cent, the concurrent loss in average yield in the us is only 10 per cent of total agricultural production. The loss is higher only in case of some highly sensitive crops like dry beans and onions in states with a warmer climate like California. In the Netherlands, where the climate is cool, yield loss due to ozone is only 3-4 per cent. While Bell says that further investigation is required to explain the difference, prima facie it appears that plant varieties in the Indian subcontinent and tropical countries are more vulnerable to ozone.
Cereal killer
Estimated crop losses due to ozone in India
Cities Loss in yield
(per cent)
Loss in yield
(thousand tonnes)


Source: M R Ashmore, Nigel Bell and others, ‘Estimating crop losses in the field,’ Imperial College, Centre for Environmental Technology, London

...And the resistant
Interestingly, ozone does not affect some plants and cultivars. Scientific evidence from field studies in Egypt show that stomata in a turnip variety almost 'waive off' ozone penetration, and in a variety of radish stomata open readily to allow ozone to pass through. Moreover, a combination of plant enzymes determines the level of resistance of a variety to ozone. Scientists feel that this knowledge could be useful in planning cultivation of crops in areas with high levels of ozone.

Overall, however, the bulk of evidence suggests that the problem of ozone impact on agriculture is no longer restricted to North America and Western Europe. It cannot, therefore, be dismissed as a problem of the North. Ozone has had a severe impact on regions and localities in countries of the South. The evidence is further reinforced by the steady increase in global and regional no x emissions, primarily from vehicles in urban areas.

Agricultural scientists are worried about the limited or poorly coordinated monitoring of air pollution in rural areas in Asia, Africa and South America. As of now, it is only possible to draw on isolated studies in a few countries that indicate the gravity of the problem. Urges Bell: "It is important to combine data on pollutants and agricultural yield in a compatible form in developing risk assessment models." He suggests appropriate use of geographical information systems in landuse planning and pollution control to minimise the adverse impact of ozone on crops.

With inputs from Priti Kumar and Shefali Verma.

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