Citizen Science has raised much interest for the last decades. In many scientific disciplines, citizens contribute to acquisition of field data mostly out of scientific interest. Institutional science used to look sceptically upon laypeople, but the attitude has largely changed as the benefits of Citizen Science for both active citizens and scientific institutions became apparent. One very successful project is SAFECAST, devoted to monitoring ambient ionizing radiation. The paper introduces the project and its measurement tool. Benefits and problems are discussed, the latter consisting primarily of uncertainty introduced by deviations from standard measurement protocol, in turn contributing to problems of interpretability. Altogether, measuring ambient dose rate is easy, but interpretation of results is not trivial and prone to spurious conclusions. One should have in mind that especially in case of real emergency (like Chernobyl and Fukushima accidents) the measurements of ambient dose-rate level only are not sufficient for proper decision-making on protective measures.

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