In the Austrian capital, amid high temperatures, animal rights campaigners demanded the city's famous horse-drawn carriages be forced to stop working when the mercury hit 30C.
Pearl Jam offered fans a full refund and said they were "very, very, deeply sorry" about the "brutal news" and "horrible timing".
"Due to the extreme circumstances at the last outdoor site outside of Paris (heat, dust and smoke from the fires) our singer Ed Vedder's throat was left damaged."
"Ed wants to play. There's just no throat available at this time."
It is not clear if Pearl Jam will finish their European tour, with final concerts planned in Prague on Friday and Amsterdam on Sunday and Monday.
On Thursday, Pope Francis called on world leaders to heed the Earth's "chorus of cries of anguish" stemming from climate change after the suffocating heatwave.
Firefighters in France's Gironde region, where more than 2000 firemen and 10 aircraft have battled for more than a week, said two huge blazes were finally contained and should be fully extinguished within weeks.
Spain, where more than 500 people died during a 10-day heatwave this month, has lost more than 1000km sq in its worst wildfires on record. As progress was made in containing the fires, the country was on alert for high temperatures.
Wildfires have already destroyed a larger area in the EU than all that was lost to blazes in the whole of last year, the bloc's satellite monitoring service said.
As of July 16, the fires had torched an area equivalent to the surface area of Trinidad and Tobago, the European Forest Fire Information System (EFFIS) said.
The worst ever recorded year for wildfires was 2017, when nearly 6000km sq were lost, but EFFIS said this year could be even worse.
Officials in Slovenia said the country faced the biggest blaze since independence in 1991 and Bosnia also battled a blaze.
The fire in Slovenia's southwestern Kras region has engulfed almost 5000 acres and set off unexploded ordnance left over from World War I.
