The warning of a high risk of high levels applies in Götaland, Svealand and the southern and central coastal areas of Norrland, according to the Swedish Museum of Natural History's website Pollen Report.
There is an increased risk in northern Norrland and parts of the central Norrland coast, as well as across large parts of Värmland, Dalarna and inland Norrland.
The risk is especially high during inclement weather.
Elsewhere in Sweden, the risk of high levels is lower, but it increases near tall grass.
Midsummer weekend is when the peak season for grass pollen begins in southern Sweden. Many tall meadow grasses bloom abundantly in these areas, according to the museum's site.
