Picocurie to Disintegrations per second

The EPA chose 4 pCi/L in 1986 as a practical action level — not a safety threshold. At the time, mitigation technology could reliably reduce levels to below 4 pCi/L but not much further. The risk at 4 pCi/L is roughly equivalent to smoking half a pack of cigarettes per day or having 200 chest X-rays per year. The EPA actually recommends considering mitigation at 2 pCi/L, but the 4 pCi/L number stuck because it was achievable and measurable with 1980s-era charcoal canisters.

Radon-222 is a gas produced by the natural decay of uranium-238 in soil and rock. Being a noble gas, it does not bind to soil particles — it seeps upward through cracks, gaps around pipes, sump pits, and any opening where the house contacts the ground. Indoor air pressure is slightly lower than soil gas pressure (the "stack effect"), so the house literally sucks radon in. A well-sealed, energy-efficient home can actually trap more radon than a drafty old one because there is less ventilation to dilute it.

Short-answer: yes, DIY kits work fine for screening. Charcoal canister tests (2–7 days, about $15) and alpha-track detectors (90 days–1 year, about $25) are available at hardware stores and by mail. You place the device in the lowest liveable area with windows closed, mail it to a lab, and get results in pCi/L. For real estate transactions, most states require a certified professional using continuous radon monitors. If your DIY test reads above 4 pCi/L, a professional follow-up is wise before spending $800–2,500 on a mitigation system.

Picocuries sound small, but they add up over decades of continuous exposure. At 4 pCi/L, you inhale about 8 radon atoms per second with each breath, 24 hours a day, for years. It is not the radon itself that does the damage — radon decays into polonium-218 and polonium-214, which are solids that lodge in lung tissue and blast it with alpha particles at point-blank range. The EPA estimates radon causes about 21,000 lung cancer deaths per year in the US, mostly among smokers where radon and tobacco synergise.

Granite contains trace uranium and therefore produces radon, but measured emission rates from countertops are typically 0.01–0.1 pCi/L contribution to room air — 10 to 100 times below the EPA action level. You would need to seal yourself in a phone booth with a granite slab to approach concerning concentrations. The radon-from-countertops scare peaked around 2008 when a few outlier samples made news, but systematic studies by the EPA and multiple universities consistently found negligible risk. Your basement floor is a vastly larger radon source.

Because dps is literally what the instrument measures — a detector counts individual nuclear decay events over time. Calling it "dps" keeps the language grounded in what physically happened. Calling it "Bq" applies an SI label to the same number. Old lab protocols, standard operating procedures written before 1975, and some US-centric equipment manuals still use dps because nobody rewrote the paperwork. Numerically, 1 dps = 1 Bq, so the conversion is trivially multiplying by one.

Counts per second (cps) is what the detector actually registers; disintegrations per second (dps) is how many decays actually occurred. No detector catches every decay — some radiation misses the detector, some is absorbed before reaching it, and some types of radiation are invisible to certain detectors. The ratio of cps to dps is the detection efficiency, which can range from under 1% (for low-energy beta emitters in a Geiger tube) to over 90% (for gamma emitters in a well counter). Getting from cps to dps requires careful calibration.

The sample is dissolved in a scintillation cocktail — a solvent containing fluorescent molecules. Each beta particle or electron excites the cocktail, producing a flash of light detected by photomultiplier tubes. But chemical impurities in the sample absorb some of that light (a phenomenon called quenching), so the counter sees fewer flashes than decays. The instrument runs an internal or external standard to measure the quench level, then applies a correction curve to convert raw cpm to true dpm, which you divide by 60 to get dps.

Not numerically — they are identical. But contextually, "dps" emphasizes the physical measurement process and appears in lab protocols where you are calculating detector efficiency: "the source emits 10,000 dps and the detector reads 3,200 cps, so efficiency is 32%." Writing that sentence with Bq would be technically correct but odd, like referring to your morning coffee temperature in kelvin. The unit name signals what kind of work you are doing.

Before the becquerel was adopted in 1975, there was no named SI unit for radioactivity — scientists just said "disintegrations per second" or used the curie. The CGPM gave the name "becquerel" to one disintegration per second to honor Henri Becquerel and to bring radioactivity into the SI naming system alongside the gray and sievert. The dps description never went away; it just lost its status as the primary label. Think of it like saying "cycles per second" instead of "hertz" — correct, but dated.