One curie of radioactivity is a sizable amount. Many times a much smaller unit is needed especially when referring to amounts contained in milk, water supplies and other common products. We could write this unit as 0.000000000001 curie, or 1 x 10^-12 curie or spell it out as one-trillionth of a cure. But that's time consuming and a heck of a lot of trouble when you're writing it fifty times a day. The shorthand version for a trillionth of a curie is generally written as 1 pCi - or even 1 pC - with the "p" standing for pico and pCi referred to as a picocurie.
Similarly, 1 becquerel is a very tiny amount of activity amounting to one radioactive disintegration per second, while we often are interested in millions or billions of decays for a single gram of a radioactive isotope. So instead of a million Bq or 10^6Bq, it is written as MBq, with the M standing for mega.
Table 4 shows prefixes and their corresponding powers of ten. Since I find that use of so many prefixes makes comparisons difficult, I'll be limiting them to as few as possible. However, others will occur in quotations and in literature you might run across.
Table 4 - International Standard (SI) Prefixes
Quintillion = 10^18 = exa = E
Quadrillion = 10^15 = peta = P
Trillion = 10^12 = tera = T
Billion = 10^9 = giga = G
Million = 10^6 = mega = M
Thousand = 10^3 = kilo = k
Hundred = 10^2 = hecto = h
Ten = 10^1 = deka = da
Tenth = 10^-1 = deci = d
Hundredth = 10^-2 = centi = c
Thousandth = 10^-3 = milli = m
Millionth = 10^-6 = micro = Greek letter mu
Billionth = 10^-9 = nano = n
Trillionth = 10^-12 = pico = p
Quadrillionth = 10^-15 = femto = f
Quintillionth = 10^-18 = atto = a
Well, we now know that radiation is caused by an atom suddenly going to pieces, but so far there is no clue as to why these particles are dangerous - if indeed they really are. So let's move on to some quantitative information about the effect of these atomic disintegrations.