Thursday was the hottest day so far this year in Switzerland, with temperatures reaching 36 degrees Celsius in Geneva.
But the rise in temperatures brought a dramatic increase in ozone concentrations, in some places measuring 198 micrograms per cubic metre – about 30 per cent above permitted levels.
New temperature highs for this year were also recorded in Chur in eastern Switzerland (35.4°C), in Sion in canton Valais (35.2°C) and in Basel (34.6°C). Temperatures hovered around a sultry 31 to 33 degrees elsewhere in the lowlands.
The hot weather attracted thousands to outdoor pools and the country’s many rivers and lakes, where water temperatures rose to 25 degrees in Lake Biel and parts of Lake Zurich.
But most waterways – many glacial-fed - remained a refreshing 19 to 22 degrees on average.
The National Air Pollution Monitoring Network said that ozone concentrations exceeded permitted levels at 14 of the 16 Swiss monitoring stations.
The record for the day was registered in Lugano in southern Switzerland (198 micrograms per cubic metre), where concentrations are often dangerously high due to the town’s proximity to the densely populated region around Milan.
This was closely followed by places north of the Alps, including Lägern (197) in the Jura hills near Zurich and on Mount Rigi (192) above Lake Lucerne.
The increased risk of respiratory infections brought on by the increased ozone levels led the Swiss weather service, MeteoSwiss, to advise people to drink plenty of water and to avoid spending too much time outdoors.
MeteoSwiss is forecasting heavy thunderstorms for later on Friday or early on Saturday, which will bring an abrupt end to the short-lived heat wave.
swissinfo with agencies
Swiss temperature records:
Highest: 41.5°C in Grono in canton Graubünden on August 11, 2003.
Lowest: -41.8 °C in La Brévine in canton Neuchâtel on January 12, 1987.
Warmest place: Locarno 11.5°C annual average.
Coldest place: Jungfraujoch, 3,454 metres above sea level, -7.9°C annual average.