Volcanic eruption at Sundhnúk crater row causes hazardous air quality in southwest Iceland
Volcanic haze and elevated levels of sulfur dioxide (SO2) from the Sundhnúk crater range eruption are affecting the western half of Iceland, especially the southwesternmost regions. The Icelandic Meteorological Office forecasts persistent volcanic haze in the south and west through Monday, due to slow and variable winds.
Air quality measurement in SW Iceland on July 21, 2025. Credit: IMO
The Icelandic Meteorological Office (IMO) reported continued gas emissions from the eruption site, with slow air movement allowing accumulation of pollutants across the southwest.
The worst-affected areas on July 21 include Reykjavík, Akranes, and surrounding settlements, where air quality monitors registered SO2 concentrations deemed hazardous to sensitive populations.
Residents are advised to keep windows closed, limit ventilation during peak pollution periods, and avoid outdoor activities. Children, the elderly, and individuals with respiratory or cardiovascular conditions are particularly at risk. The Directorate of Health recommends following official advisories and consulting its guidelines for reducing indoor air exposure.
At the Hvalfjörður, Gröf monitoring station, one of the worst-affected areas, SO2 concentrations rose sharply overnight, peaking at 1 669.0 µg/m³ at 00:00 UTC on July 21. Levels remained above 1 000 µg/m³ for several hours, including 1 415.8 µg/m³ at 01:00 UTC and 1 239.3 µg/m³ at 03:00 UTC, before gradually declining to 595.4 µg/m³ by 08:00 UTC.
The values fall within the “unhealthy” to “very unhealthy” range for sensitive individuals. PM1 concentrations also remained elevated throughout the same period, reaching up to 61.6 µg/m³.
Image credit: IMO- Image credit: IMO
- Image credit: IMO
Forecast models from the IMO show volcanic gas drifting over South and West Iceland between 10:00 LT on July 20 and 10:00 LT on July 21. Due to variable and weak wind fields, volcanic haze, comprising secondary sulfates (SO4) formed from atmospheric SO2, may persist over densely populated areas. This smog is not directly measured by sulfur dioxide sensors but can be tracked via PM1 and PM2.5 readings.
This eruptive episode started on July 16 with opening of 1 km (0.6 miles) long fissure and remained stable on July 20, with sustained activity in two central craters. Lava continues flowing eastward into Fagradalur, but cloud cover and poor visibility impede direct observation.
The current phase of the eruption has been characterized by consistent gas output and surface lava flows without major explosive behavior.
An M3.5 earthquake was recorded in Móhálsadalur, west of Kleifarvatn at 02:55 UTC on July 20. The quake was felt across the capital region and in towns such as Hveragerði.
According to IMO, this event is considered a trigger earthquake caused by stress redistribution in the crust from magma intrusions elsewhere, and does not indicate new magma movement at the epicenter.
IMO continues to monitor gas concentrations and seismicity. Residents are encouraged to report gas odors or visible haze via official reporting forms.
The Icelandic Environment Agency also provides real-time air quality data through loftgaedi.is, while health guidance is available on the Directorate of Health website.
I'm a dedicated researcher, journalist, and editor at The Watchers. With over 20 years of experience in the media industry, I specialize in hard science news, focusing on extreme weather, seismic and volcanic activity, space weather, and astronomy, including near-Earth objects and planetary defense strategies. You can reach me at teo /at/ watchers.news.
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