Billions and Billions of Molecules
If you’ve ever spent time reading about the potential effect of chemicals on your health, you’ve become familiar with the saying that “the dose makes the poison”. And you’ve typically seen dose in terms like “milligrams per kilogram”. Or if you take any medications, you’ve noted that the amount you take is expressed in milligrams (mg). For example, a standard adult dose of ibuprofen pain reliever is 400 mg.
These are expressions based on the weight of a substance, or what scientists call mass. Dose in this case is expressed as the weight of a chemical given relative to the weight of an animal or human receiving it. One of the reasons we do this is very practical—it’s easy to weigh stuff. Sometimes the weights we hear about can seem very small, so small in some cases that you can’t even see it with the naked eye. You may have heard terms like micrograms, or nanograms, or picograms. For perspective, the weight of a single grain of table salt is about 0.06 mg (or 60 micrograms).
While expressing dose in weight is convenient, it’s also important to know how many molecules there are. This is because our bodies are governed by molecules interacting with other molecules. So, it’s not surprising that toxic effects depend on the number of molecules we are exposed to and how effective each molecule is at causing an effect (often referred to as potency). As you are about to see, even something as small as a picogram (one picogram is 0.000000000000035 ounces) can be a lot of molecules.
So, how do we go from the weight of a chemical to the number of molecules of that same chemical? If you’ve ever taken a chemistry course, you may recall the concept of a “mole”. The mole is the way we convert from the weight of a chemical to the number of molecules. It’s defined by something called Avogadro’s Number (or Avogadro’s Constant). For the students of science history, it’s named after the Italian scientist Amedeo Avogadro.
A mole of any chemical is 6.02 x 1023 molecules. Yep, that’s a 6 followed by 23 zeros. It’s a LOT of molecules. To convert the weight of a chemical to the number of molecules that weight represents, we first must define two more terms–atomic weight and molecular weight.
Molecules are made of atoms and so the weight of a molecule is the sum of the weight of the atoms in that molecule. The atomic weight is the weight of one molecule of an element (all the known elements and their atomic weights can be found in what’s called The Periodic Table of the Elements). For example, water (H2O) is two atom of hydrogen and one atoms of oxygen. Oxygen has an atomic weight of 16, and hydrogen has an atomic weight of 1. So, the weight of one molecule of water (the molecular weight) is 16+1+1 = 18. What this means is that 18 grams of water (a little over a tablespoon) will be one mole, or 6.02 x 1023 molecules. Contrast that with another simple molecule like table salt, aka sodium chloride (NaCl). Table salt has a molecular weight of 23 (for sodium) + 35 (for chlorine) = 57 grams per mole (57 grams is a little over 3 tablespoons). So, compared to water, it takes almost 3 times as much table salt to equal the same number of molecules as one mole of water. Compare either of these to a mole of botulism toxin, one of the most toxic natural chemicals known. It is a large molecule with molecular weight of about 150,000. So, one mole of botulism toxin would weigh 150,000 grams, or almost 68 pounds!
Recall that all biological reactions (and toxic effects), depend on molecules interacting with other molecules. It turns out that it takes a LOT of molecules in our bodies to cause an effect. This is true for natural chemicals like hormones, as well as drugs and other man-made chemicals. To bring this to life, the table below provides perspective on the biological effect of various chemicals based on weight and the number of molecules.
Let’s look at a few examples. One grain of table salt is about 0.06 mg. Now you can see that one grain of table salt is 633,684,210,526,315,789 (633 quadrillion) molecules! What does that tell you about how many molecules of salt it takes to be able to make something taste salty? A LOT! Compare this with the number of molecules in a common dose of the pain reliever acetaminophen (chemical name for Tylenol), where one 500 mg tablet is 1,993,377,483,443,708,609,271 (about 2 sextillion) molecules. Now consider that if you took 50 mg of acetaminophen instead of 500 mg, you would likely get no pain relief. But, that 50 mg is still 1,993,377,483,443,708,609 (about 2 quintillion) molecules!
The Bottom Line
So, what’s the point of all this math and molecule mumbo jumbo? We are frequently bombarded with news that we are being exposed to dangerous chemicals, and that feels very scary. More often than not, the news is talking about an exposure to very tiny quantities. Sometimes an article may even say that there is no safe dose. However, if there was no safe dose, it would literally mean that even one molecule could cause harm. Hopefully after reading this, you can begin to understand that it takes a LOT of molecules of even the most toxic chemical to cause harm, just like it takes a lot of molecules for a chemical to provide a beneficial effect. So, the next time that you see an article that tries to make it appear that any amount of exposure is going to hurt you, take comfort in the fact that it takes a lot of molecules of even the most toxic chemical to cause harm. What you really need to know is the level to which you’re being or have been exposed, and how close that is to the level that would cause harm. Remember, the dose ALWAYS makes the poison.
| Molecular Doses of Natural and Synthetic Compounds Table | |||||
| Compound | Dose | Units (Type of Dose or Natural Level) | Molecules | ||
| 2,3,7,8 TCDD (aka dioxin) | 49 | pg/day (RfD) | 91,608,695,652 | ||
| Glyphosate | 7.0 | mg/day (RfD) | 24,934,911,242,603,600,000 | ||
| Lead
PFOA |
5.0
70 |
ug/dl (blood threshold of concern)
ng/l (EPA Health Advisory) |
14,600,000,000,000,000
101,787,439,613,527 |
||
| PFOA | 8 | ng/l (MI MCL for drinking water) | 11,632,850,241,546 | ||
| Botulism Toxin | 0.7 | ug (human lethal dose) | 2,809,333,333,333 | ||
| Tetrodotoxin (puffer fish toxin) | 16 | mg (human lethal dose) | 30,647,272,727,272,700,000 | ||
| Saxitoxin (shellfish toxin) | 14 | mg (rat oral LD50) | 28,187,290,969,899,700,000 | ||
| Sarin Gas (chemical warfare agent) | 0.7 | mg (human lethal dose) | 3,010,000,000,000,000,000 | ||
| Aflatoxin (toxin produced by some fungi) | 0.2 | mg (rat oral LD50) | 385,426,829,268,293,000 | ||
| Acetaminophen | 140,000 | mg (rat oral LD50) | 558,145,695,364,238,000,000,000 | ||
| Acetaminophen | 500 | mg (pain relief) | 1,993,377,483,443,710,000,000 | ||
| Ibuprofen | 200 | mg (pain relief) | 584,466,019,417,476,000,000 | ||
| Acetylsalicylic Acid (Aspirin) | 70000 | mg (rat oral LD50) | 234,111,111,111,111,000,000,000 | ||
| Nicotine | 3500 | mg (rat oral LD50) | 13,006,172,839,506,200,000,000 | ||
| Caffeine | 14000 | mg (rat oral LD50) | 43,443,298,969,072,200,000,000 | ||
| Ethanol | 490000 | mg (rat oral LD50) | 6,412,608,695,652,170,000,000,000 | ||
| Sugar | 5 | grams (1 teaspoon) | 8,801,169,590,643,270,000,000 | ||
| Table Salt | 0.06 | mg (about 1 grain) | 633,684,210,526,316,000 | ||
| Estradiol | 15 | pg/ml (low blood threshold for female of menstrual age) | 33,198,529,412 | ||
| Testosterone | 3 | ng/ml (low threshold for males) | 6,270,833,333 | ||
| Thyroid Hormone (T4) | 50 | ng/ml (low threshold for normal adult) | 38,738,738,738,739 | ||
Notes:
All RfD and lethal dose calculations assume a 70 kg (154 pound) person.
Blood volume = 5000 ml (about 10.5 pints)
RfD = Reference Dose. The dose which is expected to be without adverse effect even if exposed every day for a lifetime.
LD50 = Lethal Dose for 50% of the population (i.e. 50% of the population being tested would die after a single dose).
EPA Health Advisory Level: An EPA Health Advisory is a non-regulatory, non-enforceable level intended to help states and other public health officials provide a margin of protection for adverse health effects for a lifetime of exposure.
MCL: MCL stands for Maximum Contaminant Level. It is the maximum amount allowed in drinking water.
