Several days of sunshine and warm weather across most parts of Switzerland have signalled a drop in air quality, according to statistics released on Monday by the Swiss environment ministry.
Temperatures touched the 30-degree mark over the weekend, while 13 of Switzerland's 16 regional centres for measuring air quality recorded ozone levels higher than the official maximum limit.
The high temperatures over the last 10 days, coupled with minimum amounts of rainfall, have lifted the amount of ozone in the atmosphere well above the accepted level of 120 micrograms per cubic metre.
The highest nationwide recording over the weekend came from the centre in the city of Basel, which registered a level of 199 micrograms.
Despite 10 days of high temperatures and virtually uninterrupted sunshine, experts say ozone levels have yet to reach highs attained during the 1990s, when a maximum of 280 micrograms per cubic metre was recorded.
The amount of ozone in the atmosphere is currently hovering at levels which are 15 to 20 per cent less than those recorded throughout the previous decade.
Peter Strahel, a scientist based at the Swiss environment ministry, said current levels had not reached previously recorded highs because of a reduction in the use of toxic gas emissions over the past 10 years.
"The level of nitrogen oxide in the air has fallen by between 40 and 50 per cent over the last decade," Strahel said.
"But that is no reason for complacency," he added.
Statistics indicate that on a typical day in Switzerland during the summer months, around 400 tons of nitrogen oxide and 560 tons of volatile organic compounds are emitted into the atmosphere.
Canton Ticino has been worst hit by rising ozone levels over the last month. Levels have been above what is considered acceptable for 21 of the last 30 days.
Levels reached a peak in canton Ticino's largest city, Lugano, on July 3, when a figure of 241 micrograms was recorded.
Analysts have blamed the unusually high ozone levels in Ticino on industrial pollution coming from the nearby Italian city of Milan.
Concern over the increase in ozone levels has led to a greater amount of information being made available in the public domain.
Last month, Switzerland's largest telecommunications provider, Swisscom, lanched a service to provide information about ozone levels for customers in nine Swiss cantons.
Mobile phone users can use the keyword "Ozone" to receive daily updates by way of a text message on levels of ozone in selected areas across the country.
swissinfo with agencies