A post over at TeaChat lead me to this article on “Exposure and risk assessment for aluminum and heavy metals in Puerh tea” from the journal Science of the Total Environment. It looks like a very reputable journal (an impact factor of 3.190 is very good) and the bottom line of the abstract says:
However, probabilistic estimation of carcinogenic risk shows that the 95th percentile carcinogenic rate of arsenic in Puerh tea approaches the accepted risk level of 10− 4 for the highest exposure group. Therefore, the arsenic in Puerh tea is of concern.
…which leads me to dig deeper, since I’m not too keen on gobbling arsenic. The question here is what is the “highest exposure group”? Before I get to that, I’ll go through some interesting details of the article to look at their methods, so we may be able to compare what they studied with our own drinking habits. I’m going to try and leave out most of the boring statistics and health threshold information.
Basically, the authors sought to assess the health risk and exposure to heavy metals through drinking both kinds (shu and sheng) of puerh. To do this, they examined the exposure to drinkers from two cities in the region where pu’er is produced: Puer City and Kunming. They collected 36 loose puerh tea samples (17 shu, 19 sheng), 8 fresh leaf samples, and 8 soil samples. Obviously, there’s a lot of information left out here on the tea samples themselves. What factory were they made by? How old were they? How had they been stored? Was it plantation or arbor tea? Was it even pressed pu’er or was this straight maocha? There’s a lot of potential variability here that reduces the usefulness of this study to some tea drinkers, but it’s worth continuing on, because I think the sample size is large enough and they collected fresh leaf and soil samples.
One interesting tidbit about the difference between shu and sheng in the study is that the authors found that:
Concentrations of all studied elements in Puerh
fermented tea are a little higher than those in Puerh raw tea,
except for Cd.
The authors later go on to suggest that additional heavy metals are likely introduced through the fermentation process, additional processing, and water used.
To get a sense of how much of a metal may be consumed, the authors quantified how much tea was being drunk. Surveying 109 participants from Kunming and 110 from Puer City to gathered information on: amount of dry leaves, water volume per time, and number of infusions. They then used this information to calculate daily intake of dry tea leaves, which could be used to calculate how much heavy metal was being consumed. The formula is:
daily intake of dry tea leaves = daily intake of tea infusion * (amount of dry tea leaves / (water volume per time * times of infusion))
As we all know, this is a highly variable process, dependent on temperature variation, estimated timed steeped, estimated amount of dry tea leaves, and how much of the infusions actually get consumed. I’ll come back to this at the end.
The authors provided interesting statistics on how much tea was consuming in each city. Kunming drank averages of 369mL of shu and 270mL of sheng and Puer City drank averages 688mL of shu and 835 mL of sheng! However, they calculated leaf ratios per 1000mL of water and found that Kunming used 9.7g for shu and 6.8g for sheng and Puer city used 6.5g for shu and 5.6g for raw. 1000mL seems like a lot of water and some weak tea, but those are the numbers the authors provided.
Evaluating Exposure and Risk
The authors used two formulas to evaluate risk in a determinate way (using additive methods and existing data) as opposed to the second, probabilistic method they employed, which I don’t understand very well and which isn’t as useful for us, but will be mentioned below, as it was how they found supposedly alarming rates of Arsenic consumption.
The first formula was for Hazard Quotient (HQ), measured in mg/kg/day. If your HQ is less than 1, your unlikely to experience obvious adverse effects from exposure.
HQ = Average Daily Exposure / Daily Intake Reference Dose
The Average Daily Exposure (ADD) is the average daily intake of metals and can be calculated with the following formula. The Daily Intake Reference Dose comes from the EPA.
ADD = (Mean Concentration of Heavy Metals (mg/kg) * Consumption Rate (kg/day) ) / Body Weight (kg)
Theoretically, using the formula for HQ, you can calculate your own exposure risk with the following data from the study. Don’t forget to convert grams consumed into kilograms! Tables appear to be a challenge in WordPress, so please excuse the three long lists.
Reference Dose (mg/kg/day)
Study Findings: Fermented (mg/kg)
Study Findings: Raw (mg/kg)
For example, if I wanted to calculate my risk of exposure to Arsenic (As), based on the 7.5 g of sheng (converted to 0.0075 kg) I drank yesterday, I could do the math and find that my HQ for that metal is 0.00000006. Pretty safe. It helps that I’m not a small man.
They used a second equation, Hazard Index, which adds together the HQs for all the metals. The authors say:
In summary, even for the exposure group with the
largest HI value, the HI value was below 1, indicating that there is no significant non-carcinogenic health risk from metals due to drinking of Puerh tea.
You’ll probably note the usage of “non-carcinogenic.” Metals can also have a cancer-risk associated with them. This is where the concern for Arsenic comes in. Their more advanced probabilistic method indicated that Arsenic may have been above safe levels for men living in Puer City, who consumed 11.26 g/day. All research, however, uses assumptions that may not be correct.
Exceptions and Limitations
The authors note:
all aforementioned results in this paper are based on the
assumption that tea drinkers absorb all metals contained in the
tea leaves. In reality, most people drink the tea infusion and discard the tea leaves, and only some of the metal present dissolves in the water.
This assumption that 100% of the metal makes it into the drinker is completely false. They discuss transfer rates for infusion, with each metal having a different rate, most based on green tea studies and indicating high variability dependent on brewing time, temperature and pH. They suggest that Arsenic’s transfer rate may be around 23.8%-32.9%. With this in mind, they say that:
the risk of As in Puerh tea is thought to be less than one third
of that estimated above.
Looking at the other metals, they also note that rinsing the tea reduces the risk to some metals even more significantly:
In our survey, 89.6% of the inhabitants of Kunming and 93.3% of the inhabitants of Puer had the custom of washing tea by discarding the quickly brewed first infusion and drinking tea from the second infusion. This practice should further lower the risk of exposure to Al, Cu and Zn.
At this point, I’m not too worried anymore and think the the authors maybe shouldn’t even be making claims about potential carcinogenic Arsenic problems in pu’er without accounting for these variability in transfer rates and actual consumption of the heavy metals. An interesting bottom line, however, is that you should always rinse your pu’er and don’t eat the leaves if you’re worried about heavy metals!
Hongbin Cao, Li Qiao, Hui Zhang, Jianjiang Chen, Exposure and risk assessment for aluminium and heavy metals in Puerh tea, Science of The Total Environment, Volume 408, Issue 14, 15 June 2010, Pages 2777-2784.