How Much Arsenic Do We Eat? Analytical chemists make light work in tracking potentially harmful chemicals Julian Tyson Department of Chemistry UMass Amherst
Analytical chemists provide information about the chemical composition of materials. So that a decision can be taken. How do analytical chemists make light work in tracking potentially harmful chemicals? Interaction of light with materials.
Light is a wave and is a form of energy. The energy of light is inversely related to its wavelength. Short wavelength: high energy. Long wavelength: low energy
Summary of where we are now. Color of objects is due to the absorption of light. Absorption is due to the molecules. The wavelengths of the light absorbed tell us something about which chemicals are present. Amount of light absorbed depends on the number of molecules in the light beam, so The extent of the absorption tells us something about how much of a chemical is present.
Analytical chemists use the interaction of light with with molecules that contain arsenic to measure the arsenic in relevant materials.
So how does arsenic get into relevant materials? Arsenic occurs in nature. It is about the 40th most abundant element on the planet. Geological processes have spread it around. We’ve extracted it from ores, made useful compounds and spread them around. Open ocean water: about 1 - 4 ppb River and surface water: about 0.1 - 1 ppb
The arsenic in the environment story Orpiment As2S3 Realgar As4S4 Arsenic trioxide As2O3 Arsenic As
Ground water: a major problem around the world. Especially in SE Asia (Bangladesh and West Bengal, India) Concentrations: 0.1 – 1000 ppb. US EPA limit is 10 ppb
Concentrations in soil are about 2 mg kg-1. But there are considerable variations and there’s a legacy of previous agricultural practice to deal with.
Amounts. Weight: 1 kilogram (kg) = 2.2 lb = 1,000 grams (g) 1 milligram (mg) = one-thousandth of a gram 1 microgram (µg) = one-millionth of a gram. A grain of salt weighs 2 mg (the official grain is 65 mg) Amounts. Volume: liter (L) = 0.26 gallon (US) = 1,000 milliliters (mL) 1 milliliter (mL) = one-thousandth of a liter 1 microliter (µL) = one-millionth of a liter A drop (of water) is about 50 µL A bathtub contains about 230 L
Concentrations in solids: mass per mass 1 part per million (ppm) is 1 milligram in 1 kilogram i.e. 1 mg/kg or 1 microgram in 1 gram i.e. 1 µg/g Scientist sometimes write this as 1 µg g-1 A part per billion (ppb) is 1 µg kg-1 Concentrations in liquids: mass per volume 1 part per million (ppm) is 1 milligram in 1 liter i.e. 1 mg/L or 1 microgram in 1 mL i.e. 1 µg/mL Scientist sometimes write this as 1 µg mL-1 A part per billion (ppb) is 1 µg L-1 1000 ppb = 1 ppm
How to make a 10 pbb solution One grain weighs 65 mg (2.5 mm cube of As2O3) 76% by weight is As 49 mg 21 bathtubs contain 21 x 233 L = 4,900 L 1 grain in 21 bathtubs = 10 ppb The US EPA limit for arsenic in drinking water
Arsenic speciation There are a lot of different arsenic compounds in the environment. Inorganic Arsenic bonded to oxygen and/or sulfur or hydrogen. As2O5 H3AsO4 arsenate As2O3 H3AsO3 arsenite AsH3 arsine Organic Arsenic bonded to carbon and/or oxygen or hydrogen Methylated arsenate CH3AsO(OH)2 MMA (CH3)2As(O)OH DMA Arsenobetaine, arsenocholine
Some formulae for the chemists in the audience. arsenate monomethylarsonate arsenite cacodylate trimethylarsine oxide dimethylarsinate
More formulae for the chemists in the audience. trimethylarsine oxide tetramethylarsonium
Yet more formulae for the chemists in the audience. TETRA arsenobetaine Seafood arsenic arsenocholine
Not all arsenic compounds are very toxic OK - all chemicals are toxic: it all depends on the dose and how you interact with them. Some arsenic compounds are “non-toxic.” arsenobetaine and arsenocholine and arsenosugars seafood arsenic Some arsenic compounds are toxic Acute lethal dose of As2O3 is about 100 mg for a 70-kg adult. About 2 grains or a 3 mm cube. Some arsenic compounds are sort of non-toxic Monomethyl and dimethyl arsenates The adverse health effect of concern is cancer.
How does arsenic get into our food? Plants take up arsenic compounds from the soil. Legacy of previous applications.
How do we know whether food and drink contains potentially harmful forms of arsenic? • Ask the analytical chemists. They tell us. • Seafood contains non-toxic forms • Drinking water contains toxic, inorganic forms • Food contains toxic (inorganic) and sort-of toxic forms (mono- and di- methylated). • Approach: Measure total arsenic and assume it is all toxic. • If less than guideline value: no problem • If more than guideline value: more analysis needed
How much arsenic is considered safe? How much inorganic arsenic is considered safe? Food: No US guideline value. Drinking water: US EPA 10 µg L-1 (ppb) Joint FAO/WHO expert committee on food additives, seventy-second meeting, Rome, 16–25 February 2010, summary and conclusions Issued 16th March 2010 http://www.who.int/foodsafety/chem/summary72_rev.pdf The limit for dietary inorganic arsenic exposure is: 3.0 µg per kg body weight per day
How much inorganic arsenic is considered safe? Dietary: 3.0 µg per kg body weight per day. Acute lethal dose (adult) 100 mg
How much inorganic arsenic do we actually eat? Food: Ask the analytical chemists. Acute lethal dose (adult) 100 mg
A market basket survey of inorganic food in rice Food and Chemical Toxicology 37 (1999) 839-846 Examined 40 commodities expected to deliver 90% of total dietary inorganic arsenic. Took 4 samples of each. Measured total arsenic and inorganic arsenic.
Environmental Health Perspectives, 2010, 118, 345-350 Probabilistic modeling of dietary arsenic exposure and dose and evaluation with 2003-2004 NHANES Data Scientists at US EPA, National Taiwan University, and the Environmental and Occupational Health Sciences Institute Inorganic arsenic. 12 11 24 18 17 13 5
Environmental Health Perspectives, 2010, 118, 345-350 Results: Mean total exposure from food is 0.38 µg per kg body weight per day. 14-times higher than the mean exposure from drinking water. Exposure from food is 0.05 µg per kg body weight per day i.e. 1.96 µg per day. Approximately 10% of exposure from food is the toxic inorganic form of arsenic. The average American weighs 40 kg i.e. 88 lbs.
Calculated exposure from food. Total arsenic: µg per kg body weight per day
Calculated exposure from food. Inorganic arsenic: µg per kg body weight per day
The Schoof report: A market basket survey of inorganic food in rice There’s a problem with the Schoof analysis of the rice.
What do other researchers find about arsenic in rice? Food Additives and Contaminants: Part B, 2009, 2, 112-120 Survey of total arsenic and arsenic speciation in US-produced rice as a reference point for evaluating change and future trends. Heitkemper and colleagues at USFDA Cincinnati The average total content was 210 µg kg-1. Regardless of total content, inorganic arsenic rarely exceeded 150 µg kg-1. Arsenic content in US rice has been relatively constant over the past 30 years.
All values in µg kg-1 based on dry weight. *The minus sign means that the sum of I + MMA + DMA > total #One sample had over 1000 µg kg-1 total: inorganic 150 µg kg-1
Heitkemper and colleagues at US FDA Cincinnati Conclusions “In comparison with inorganic arsenic, much less is known regarding the toxicity and biotransformation pathways associated with dietary intake of DMA. Although generally considered to be less toxic than inorganic arsenic, more information about the long-term exposure effects of DMA may be necessary to evaluate fully the risk of rice consumption, given the high DMA content that has been reported in rice.”
What do other researchers find about arsenic in rice? Meharg and colleagues at the University of Aberdeen
Meharg and colleagues at the University of Aberdeen Conclusions “The modeling outlined here indicates that eating rice at typical SE Asia consumption rates, or at higher percentile consumption rates in developed countries, constitutes a significant excess cancer risk to those populations, well above the targets set by US EPA and WHO for carcinogenic sources.” “As cancers caused by chronic exposure to arsenic sources have a latency of approx. 20 years, the data presented here suggest that long-term epidemiological studies need to be undertaken to characterize this risk.”
Researchers call for monitoring of arsenic levels in rice By Elizabeth Weise, USA TODAY Dec 5, 2011 Proceedings of the National Academy of Sciences of the USA Dec 5th 2011 Rice consumption contributes to arsenic exposure in US women Diane Gilbert-Diamond and coworkers at Dartmouth College “Emerging data indicate that rice consumption may lead to potentially harmful arsenic exposure.” “Although inorganic arsenic is thought to be more harmful that DMA, further epidemiological studies are needed to better understand the health risks of DMA . . . It is imperative that the health impact of arsenic exposure through rice consumption be characterized.” www.pnas.org/cgi/doi/10/.1073/pnas1109127108
How Much Arsenic Do We Eat? The question really should be: How much inorganic arsenic and methylated arsenic do we eat? Answer: depends on what foods we eat. Rice has a much higher concentration than any other food. What sort of rice, where from and how much. White rice contains less arsenic than brown rice. Where grown. USA, France, and Spain (max total) China, Bangladesh and Italy (max inorganic). Bioavailable? Assume 100%.
There’s no federal limit for arsenic in juice. 25% of samples exceeded 5 ppb and 10% exceeded 10 ppb. Recommended dietary limit is 3 µg per kg body weight per day. 30 - 60 µg for young children. Need to drink 1.2 – 2.4 L i.e. 2.5 – 5 pints per day.