Melilla, a Spanish city on the North African coast, has emerged from 15 years of monitoring as a place with lower natural radiation than Spanish and global averages.
That result redraws the city’s risk map, turning one volcanic corner into the exception that explains the whole.
Quiet city map
Across 27 soil samples and 47 on-site dose measurements, one pattern kept repeating in Melilla.
Using that evidence, physicist Jesús García Rubiano at the University of Las Palmas de Gran Canaria (ULPGC) traced the calmest readings northward.
Higher values clustered near the airport and southwestern edge, where volcanic rocks naturally carry more uranium, thorium, and potassium.
The north-south contrast kept the citywide average low, while singling out the smaller places where extra testing matters.
Radiation in Melilla soil
In the soil itself, the team measured radioisotopes, unstable forms of atoms that release radiation as they decay.
Average concentrations reached 22, 24, and 305 decays each second in every 2.2 pounds of soil.
These soil values sat below worldwide medians and below Spanish averages, which means the ground generally emitted less natural radiation than expected.
Even so, the soil totals do not erase local hot spots, but they explain why Melilla’s broader picture stayed unusually calm.
Why rocks matter
Beneath Melilla, two kinds of ground drove the split between quiet neighborhoods and stronger readings.
Carbonate and river-laid sediments usually held lower amounts, because they contain fewer heavy elements that feed natural decay chains.
Southwestern volcanic deposits pushed values upward, especially where potassium-rich volcanic rock crops out near the airport.
Geology, not some hidden accident, explains why one end of the city registers a little more radiation than another.
Traces from fallout
One of the clearest clues did not come from local rock at all, but from cesium-137, a radioactive trace left by past fallout.
Northern forested ground held more of it, likely because less digging and less erosion let those particles remain in place.
A second tracer, lead-210, followed much the same pattern while also rising near industrial areas and fuel depots.
Fallout marks matter less as a health warning than as a fingerprint of how wind, soil loss, and human activity move material around.
Melilla beaches set radiation baseline
Along Melilla’s beaches, readings dropped close to the background from cosmic rays, giving researchers a clean coastal baseline.
Some artificial stretches still varied, because imported sand and leftover landfill material can carry different mineral mixes.
Baseline work matters in a port city near the Strait of Gibraltar, where heavy ship traffic and regional industry complicate future contamination alerts.
With a baseline in hand, ULPGC researchers could spot later spikes instead of folding them into ordinary coastal variation.
Dose over years
Turning soil readings into gamma radiation, high-energy rays released during atomic decay, the team still found a low average outdoor dose.
Measurements at 47 sites produced a mean terrestrial dose rate of 47 nanograys per hour, though volcanic spots rose to 172.
Melilla’s estimated annual effective dose came to 0.23 millisieverts, and the lifetime cancer risk stayed below global averages.
Those low averages make the survey read less like an alarm than a guide to where closer checks belong.
Radon follows ground
Indoors, the more serious question involves radon, a radioactive gas that seeps upward from soil and can collect inside buildings.
A European directive put the indoor radon reference at 300 decays each second in about 35 cubic feet of air.
In Melilla, 70.5 percent of the urban area fell into the lowest category, leaving roughly 2,787 buildings in the watch zones.
That split does not prove high indoor levels, but it marks where testing should happen first.
What the map misses
Maps like this are powerful, but they do not measure what any one family actually breathes inside a room.
Ventilation, cracked floors, construction style, and daily habits can raise or lower indoor buildup even on the same street.
Once inhaled, radon decay particles can damage DNA in airway cells, which is why the gas matters far beyond geology.
Health agencies estimate that radon causes between three and 14 percent of lung cancers, and smokers face 25 times the risk.
Why Melilla radiation matters
For a compact city pressed between sea, border crossings, and older construction, a trustworthy radiation map is more than curiosity.
Years before this paper, local reporting already described Melilla as having some of Spain’s lowest natural radiation levels.
Natural radiation remains a constant background feature of the environment, present in soil, air, and building materials.
That understanding now rests on a detailed map that helps inspectors, planners, and residents focus on the southwest.
After 15 years of sampling, Melilla emerges as a place with low average environmental radiation and sharply local geology-driven exceptions.
The next step is simple: measure inside the southwestern hot spots, refresh the beach baseline, and let the ULPGC survey guide action.
The study is published in Nature.
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