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We propose hypothesis accounting for the radiocarbon content increase in the terrestrial atmosphere around 5480 BC. It differs from similar increases caused by solar activity depressions by time profile. This time behaviour may be explained if one assume the Solar system collision with compact (d  ∼ 10–100 au) and dense (n  ∼ 10–1000 cm−3 and more) interstellar cloud. We also suggest experiments on the 3He/4He ratio measurements in atmospheric gases captured in the polar ice cores to check this idea because interstellar cloud is characterized by two orders of magnitude greater ratio than the Earth's atmosphere. [ABSTRACT FROM AUTHOR]