Tag: excretion

Can we be sure that fun in the sun alters water chemistry?

Minimalist sampling and experimental variables

Keith S. Taber

Dirty water

I was reading the latest edition of Education in Chemistry and came across an article entitled "Fun in the sun alters water chemistry. How swimming and tubing are linked to concerning rises in water contaminants" (Notman, 2023). This was not an article about teaching, but a report of some recent chemistry research summarised for teachers. [Teaching materials relating to this article can be downloaded from the RSC website.]

I have to admit to not having understood what 'tubing' was (I plead 'age') apart from its everyday sense of referring collectively to tubes, such as those that connect Bunsen burners to gas supplies, and was intrigued by what kinds of tubes were contaminating the water.

The research basically reported on the presence of higher levels of contaminants in the same body of water at Clear Creak, Colorado on a public holiday when many people used the water for recreational pursuits (perhaps even for 'tubing'?) than on a more typical day.

This seems logical enough: more people in the water; more opportunities for various substances to enter the water from them. I have my own special chemical sensor which supports this finding. I go swimming in the local hotel pool, and even though people are supposed to shower before entering the pool: not everyone does (or at least, not effectively). Sometimes one can 'taste' 1 the change when someone gets in the water without washing off perfume or scented soap residue. Indeed, occasionally the water 'tastes' 1 differently after people enter the pool area wearing strong perfume, even if they do not use the pool and come into direct contact with the water!

The scientists reported finding various substances they assumed were being excreted 2 by the people using the water – substances such as antihistamines and cocaine – as well as indicators of various sunscreens and cosmetics. (They also found higher levels of "microbes associated with humans", although this was not reported in Education in Chemistry.)

I'm not sure why I bother having a shower BEFORE I go for a swim in there… (Image by sandid from Pixabay)

It makes sense – but is there a convincing case?

Now this all seems very reasonable, as the results fit into a narrative that seems theoretically feasible: a large number of people entering the fresh water of Clear Creek are likely to pollute it sufficiently (if not to rename it Turbid Creek) for detection with the advanced analytical tools available to the modern chemist (including "an inductively coupled plasma mass spectrometer and a liquid chromatography high resolution mass spectrometer").

However, reading on, I was surprised to learn that the sampling in this study was decidedly dodgy.

"The scientists collected water samples during a busy US public holiday in September 2022 and on a quiet weekday afterwards."

I am not sure how this (natural) experiment would rate as a design for a school science investigation. I would certainly have been very critical if any educational research study I had been asked to evaluate relied on sampling like this. Even if large numbers of samples were taken from various places in the water over an extended period during these two days this procedure has a major flaw. This is because the level of control of other possibly relevant factors is minimal.

Read about control in experimental research

The independent variable is whether the samples were collected on a public holiday when there was much use of the water for leisure, or on a day with much less leisure use. The dependent variables measured were levels of substances in the water that would not be considered part of the pristine natural composition of river water. A reasonable hypothesis is that there would be more contamination when more people were using the water, and that was exactly what was found. But is this enough to draw any strong conclusions?

Considering the counterfactual

A useful test is to ask whether we would have been convinced that people do not contaminate the water had the analysis shown there was no significant difference in water samples on the two days? That is to examine a 'counterfactual' situation (one that is not the case, but might have been).

In this counterfactual scenario, would similar levels of detected contaminants be enough to convince us the hypotheses was misguided – or might we look to see if there was some other factor which might explain this unexpected (given how reasonable the hypothesis seems) result and rescue our hypothesis?

Had pollutant levels been equally high on both days, might we have sought ('ad hoc') to explain that through other factors:

  • Maybe it was sunnier on the second day with high U.V. levels which led to more breakdown of organic debris in the river?
  • Perhaps there was a spill of material up-river 3 which masked any effect of the swimmers (and, er, tubers?)
  • Perhaps rainfall between the two sampling dates had increased the flow of the river and raised its level, washing more material into the water?
  • Perhaps the wind direction was different and material was being blown in from nearby agricultural land on the second day.
  • Perhaps the water temperature was different?
  • Perhaps a local industry owner tends to illegally discharge waste into the river when the plant is operating on normal working days?
  • Perhaps spawning season had just started for some species, or some species was emerging from a larval state on the river bed and disturbing the debris on the bottom?
  • Perhaps passing migratory birds were taking the opportunity to land in the water for some respite, and washing off parasites as well as dust.
  • Perhaps a beaver's dam had burst up stream 3 ?
  • Perhaps (for any panspermia fans among readers) an asteroid covered with organic residues had landed in the river?
  • Or…

But: if we might consider some of those factors to potentially explain a lack of effect we were expecting, then we should equally consider them as possible alternative causes for an effect we predicted.

  • Maybe it was sunnier on the first day with high U.V. levels which led to more breakdown of organic debris in the river?
  • …
  • Perhaps a local industry owner tends to illegally discharge waste into the river on public holidays because the work force are off site and there will be no one to report this?
  • … etc.

Lack of control of confounding variables

Now, in environmental research, as in research into teaching, we cannot control conditions in the way we can in a laboratory. We cannot ensure the temperature and wind direction and biota activity in a river is the same. Indeed, one thing about any natural environment that we can be fairly sure of is that biological activity (and so the substances released by such activity) varies seasonally, and according to changing weather conditions, and in different ways for different species.

So, as in educational research, there are often potentially confounding variables which can undermine our experiments:

In quasi-experiments or natural experiments, a more complex design than simply comparing outcome measures is needed. …this means identifying and measuring any relevant variables. …Often…there are other variables which it is recognised could have an effect, other than the dependent variable: 'confounding' variables.

Taber, 2019, p.85 [Download this article]
independent variableclass of day (busy holiday versus quiet working day)
dependent variablesconcentrations of substances and organisms considered to indicate contamination
confounding variablesanything that might feasibly influence the level of concentrations of substances and organisms considered to indicate contamination – other than the class of day
In a controlled experiment any potential confounding variables are held at fixed levels, but in 'natural experiments' this is not possible

Read about confounding variables in research

Sufficient sampling?

The best we can do to mitigate for the lack of control is rigorous sampling. If water samples from a range of days when there was high level of leisure activity, and a range of days when there was low level of leisure activity were compared, this would be more convincing that just one day from each category. Especially so if these were randomly selected days. It is still possible that factors such as wind direction and water temperature could bias findings, but it becomes less likely – and with random sampling of days it is possible to estimate how likely such chance factors are to have an effect. Then we can at least apply models that suggest whether observed differences in outcomes exceed the level likely due to chance effects.

Read about sampling in research

I would like to think that any educational study that had this limitation would be questioned in peer review. The Education in Chemistry article cited the original research, although I could not immediately find this. The work does not seem to have been published in a research journal (at least, not yet) but was presented at a conference, and is discussed in a video published by the American Chemical Society on YouTube.

"With Labor Day approaching, many people are preparing to go tubing and swimming at local streams and rivers. These delightful summertime activities seem innocuous, but do they have an impact on these waterways? Today, scientists report preliminary [sic] results from the first holistic study of this question 4, which shows that recreation can alter the chemical and microbial fingerprint of streams, but the environmental and health ramifications are not yet known."

American Chemical Society Meeting Newsroom, 2023

In the video, Noor Hamdan, of John Hopkins University, reports that "we are thinking of collecting more samples and doing some more statistical analysis to really, really make sure that humans are significantly impacting a stream".

This seems very wise, as it is only too easy to be satisfied with very limited data when it seems to fit with your expectations. Indeed that is one of the everyday ways of thinking that science challenges by requiring more rigorous levels of argument and evidence. In the meantime, Noor Hamdan suggests people using the water should use mineral-based rather than organic-based sunscreens, and she "recommend[s] not peeing in rivers". No, I am fairly sure 'tubing' is not meant as a euphemism for that. 5

Work cited:

Notes:

1 Perhaps more correctly, smell, though it is perceived as tasting – most of the flavour we taste in food is due to volatile substances evaporating in the mouth cavity and diffusing to be detected in the nose lining.

2 The largest organ of excretion for humans is the skin. The main mechanism for excreting the detected contaminating substances into the water (if perhaps not the only pertinent one, according to the researchers) was sweating. Physical exertion (such as swimming) tends to be associated with higher levels of sweating. We do not notice ourselves sweating when the sweat evaporates as fast as it is released – nor, of course, when we are immersed in water.

One of those irregular verbs?

I perspire.

You sweat.

She excretes through her skin

(Image by Sugar from Pixabay)

3 The video suggests that sampling took place both upriver and downriver of the Creek which would offer some level of control for the effect of completely independent influxes into the water – unless they occurred between the sampling points.

4 There seem to be plenty of studies of the effects of water quality on leisure use of waterways: but not on the effects of the recreational use of waterways on their quality.

5 Just in case any readers were also ignorant about this, it apparently refers to using tyre inner tubes (or similar) as floatation devices. This suggests a new line of research. People who float around in inner tubes will tend to sweat less than those actively swimming – but are potentially harmful substances leached from the inner tubes themselves?

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Is 6% kidney function just as good as 8% kidney function?

A case of justifying dubious medical ethics by treating epistemology as ontology

Keith S. Taber

Image by Mohamed Hassan from Pixabay

I was puzzled by something I heard a hospital doctor say regarding kidney functioning. The gist of his comments were that

  • once kidney function was below about 10% of normal functioning…
  • then protecting remaining kidney function was not important…
  • because estimates of function at that level are unreliable.

I thought this was an illogical argument as it confused ontology (the state of the kidneys and their functioning) and epistemology (how well we can measure kidney function).

The kidneys are essential organs that regulate hydration levels and eliminate toxic materials from the body. They are 'essential' in the sense that without kidney function someone soon dies. Typically healthy people have plenty of scope for contingency in the capacity of their kidneys. (Living kidneys donors give up one of their two kidneys for transplantation, so, after donation, they will only have, at best, 50%,of normal functioning.) So when people's kidneys start to deteriorate due to disease the patient can continue with normal life for some time. I am not an expert, but from what I understand, a person can manage a normal life with 20% of normal functioning.

Of course there reaches a point in progressive kidney disease when the remaining capacity is not enough to keep someone alive for an extended period. So if kidney function drops to something like an eighth of normal healthy functioning, the situation gets critical.

Kidney dialysis

These days people can have dialysis if their kidneys fail. Someone with 0% kidney function – someone who never excretes any urine at all – can be kept alive indefinitely by dialysis. However this is not ideal. The patient has to attend a clinic and have treatment for 3-4 hours at a time, usually three times a week. No time off – no holidays from dialysis if the patient wants to continue living (and some decide they would rather not continue living, although most 'tolerate' the treatment). Often patients feel unwell on, or after, dialysis – they may say they feel 'washed out', for example. Dialysis also costs the health service (or in some countries, the patient) a good deal of money.

Dialysis patients also have to be very careful about diet and avoid some foods (e.g., eating bananas can lead to dangerously high levels of potassium that can interfere with heart function and could lead to a heart attack), as sessions of dialysis (with no, or very little, blood filtration occurring in-between) is never as good as having constantly functioning kidneys.

Then there's the problem of fluid intake

Dialysis patients are asked to limit their intake of fluids. A healthy person who drinks a lot (whether tap water, tea, beer, etc.) simply produces more urine. Most dialysis patients, however, produce little, if any, urine, and the difference between what they 'should' excrete (to maintain homeostasis), and what they can actually excrete, needs to be removed during the dialysis process. So, whatever water a patient takes in drinks during the 45 or so hours between sessions (and is not lost through some other mechanism such as sweating or breathing), is all taken off during three or so hours on the machine. This brings about changes in the blood volume much more quickly than is comfortable. As the body cannot remove excess fluid via the kidneys, fluid intake means the fluid levels build up between dialysis sessions which can lead to various complications such as increases in blood pressure.

Dr McCoy is unimpressed by 20th Century medicine (Star Trek IV: The Voyage Home, Paramount Pictures)

So, having kidney function of, say, 10% or less of normal is a real pain and requires reorganising your entire life around your dialysis sessions (or perhaps getting a transplant if you are strong enough for surgery and are lucky enough that a good match can be found).

That provides some background in considering whether, once kidneys have deteriorated below, say 10%, it really makes any difference in worrying about the actual level. If you have 8% of normal functioning and are on dialysis for life, why would it matter if that fell to 6%?

An actual case

The context of this question was a patient with kidney failure or end-stage renal disease (a haemodialysis* patient, who would only live a matter of days without regular treatment) who was given a CAT scan** using a contrast medium*** to show up features that would not be observable otherwise. Such media are widely considered to have some toxicity in relation to the kidneys (Ahmed, Williams & Stott, 2009), but in a healthy person they are eliminated through the kidneys quite quickly and any risk is considered small. A person with kidney failure does not eliminate toxins in this way, and so when a scan is indicated, it can be scheduled for just before their next dialysis session.

"In every study comparing patients with and without some degree of renal insufficiency [kindeys not functioning adequately], renal insufficiency increased the likelihood of RCIN [radiocontrast-medium-induced nephropathy, i.e., kidney damage due to the use of contrast media]"

"Both peritoneal and hemodialysis remove substantial amounts of the contrast medium (50% to 90% of the dose); hemodialysis is more effective."

Solomon, 1998: 230, 236.

This patient, however, was admitted to a hospital very ill. The emergency department doctor ordered an immediate scan – late at night, at a weekend – but told the patient that the on-call dialysis staff could be called in to give dialysis after the scan. At the X-ray department, the radiographer then said that this was not needed, as long as the patient had dialysis within 24 hours of the scan.

The renal doctor's viewpoint

The next afternoon, the patient had still not gone for dialysis when the hospital renal doctor visited the patient. This doctor took the view that as the patient was due their regular dialysis the following day (i.e., about 38 hours after the scan), there was no point sending the patient for an additional dialysis session, as – after all – the kidneys had already failed sufficiently for the patient to be relying on dialysis for survival.

The patient's viewpoint

The counter-argument presented to the renal specialist (by the patient's spouse) was that even at this point further deterioration should be avoided if possible – that even if 8% of normal kidney function was not good, it was inherently better than 6% of normal kidney function.

After all, if for some reason a patient was further compromised (by an unrelated illness, or by delay in accessing normal dialysis due to some unexpected contingency) a few percentage points – making a small difference in how much the body could remove toxins and excess fluid from the blood by itself between dialysis sessions – could still be the critical factor in determining whether the person survived. (Those attending hospital dialysis notice the high frequency of fellow regular patients who, suddenly, are no longer attending for treatment.)

The renal doctor's justification

The doctor responded to this with the counter-argument that once kidney function was this low, there was no reason to be concerned about a change in measured kidney function from (say) 8% to 6% as the difference between such measurements was within the usual variations in measurements found in patients from time to time.

There are two issues here of interest.

Consent that is conditional is not consent if the conditions are broken

One issue relates to ethics (here, medical ethics). A patient consented to a diagnostic procedure with a possible risk of side effects on the understanding that a suitable counter measure would be taken immediately after the procedure to minimise any detrimental effect. The hospital undertook the procedure, but then decided (when it was too late for the patient to withdraw consent) not to follow through on the promised counter-measure. In effect, a procedure was carried out without consent as the consent was (as was made absolutely clear by the patient) conditional on the scan being followed by dialysis.

Reasons for refusing to provide treatment

The second issue relates to the justification given by the doctor as reported above.

The day after the explanation about measurement not clearly distinguishing between 8% and 6% functioning had been made, when dialysis was finally provided, another renal specialist offered a different justification entirely – that the potential risk to kidneys of the contract medium was just a myth. However, the earlier conversations

  1. in the emergency department;
  2. in the X-ray department; and
  3. with the first renal doctor within 24 hours of the scan,

were all clearly undertaken on the basis that both patient and medical staff thought the contrast medium was potentially damaging to kidneys.

"These contrast media can occasionally cause kidney damage, especially in patients who already have kidney disease"

Ahmed, Williams & Stott, 2009

In the context of that discourse, the first renal specialist had argued that because (a) the precision of estimates of kidney function was not great enough to reliably measure a difference between 6% and 8% functionality, then (b) there was no need to be concerned about treatment which could potentially cause damage bringing about deterioration of this order.

Presumably,

  • at any one time, a person's kidney function will be at a certain level.
  • If the kidney is then further damaged by toxins then that functionality will drop.
  • A more damaged kidney is inherently less desirable than a less damaged (better functioning) kidney.
  • So further damage to an already damaged kidney is inherently undesirable,
  • and should be avoided if possible, if the costs of doing so are not too high.

The state of a diseased person's kidneys could vary slightly 'naturally' in response to various factors related to their general health, diet, environment, etcetera. This is an ontological consideration – the actual state of the kidneys changes. This may well mean that changes of a few percent between measurements could just be natural fluctuation.

It may therefore be difficult to tell if a person's kidneys have become more damaged due to a particular event, such as a diagnostic scan. That is an epistemological issue – the limitation on how well we can identify a specific change that is masked by noise.

Presumably, there are also various factors that limit the precision of such estimates – all measurements are subject to errors, and small (real) differences may be difficult to identify if they are at the level of the likely measurement error. That is also an epistemological issue.

But, just because an effect cannot be clinically measured (epistemology), that does not mean it is not real and will not have consequences (ontology). A drop from 8% kidney function to 6% kidney function is only a change of 2% compared with normal functioning, BUT it is a loss of 25% of the patient's actual kidney function.

A small deterioration in already severely compromised kidneys may seem insignificant to the renal doctor because he does not think he could reliably measure the change. One day it could be the difference between life and death to the kidneys' owner.

Sources cited:
  • Ahmed, A., Williams, G., & Stott, I. (2009). Patient information-What I tell my patients about contrast medium nephrotoxicity. British Journal of Renal Medicine, 14(3), 15-18.
  • Solomon, R. (1998). Contrast-medium-induced acute renal failure. Kidney international, 53(1), 230-242.

* haemodialysis involves the patient having permanent 'plumbing' installed that allows their vascular system to be connected to a dialysis machine, so the blood can be diverted to the machine to be cleaned. This usually done using blood vessels in the arm. In the case discussed the surgeon cut into the neck and chest (with the patient fully conscious), and connected tubing to a vein in the neck. The tubing was run beneath the skin to exit in the chest below the neckline, where a fitting acted as a tap and connector for the external tubing to the machine. Very special care has to be taken to keep the area clean, and the dressing dry, as the plumbing provides a direct route into the bloodstream. (Baths, swimming, hot-tubs, etc. are not advisable.)

[Peritoneal dialysis is an alternative treatment that involves a catheter being implanted in the abdomen, and being used to allow a solution into the abdominal cavity, which is later removed after it has absorbed waste materials. The patient can manage the process at home, but needs to change the solution in the abdomen a number of times each day.]

** computerised tomography: a process that uses a series of X-ray bursts to collect data that can be compiled into a 3-D image.

*** a substance that shows up on X-ray scans, and which when injected into the blood helps detect vascular structures. (The term is generic – it also applies to substances swallowed  before scans of the alimentary canal.)

Note: this post was originally prepared in October 2015, but was not published at the time when the patient was alive and attending for treatment.