There is no generic answer to your question: the best sample depends on the exposure pattern and on the metal of interest - and also of the time-span of the exposure and of interest.

Hair is be appropriate for mercury exposure, when the species of interest is methyl mercury (= dietary exposure mainly from fish), and the interest is long term exposure or changes of exposure with time (analysis of different parts of hair) but it is not suitable for the assessment of exposure to metallic or inorganic mercury species especially at work place, where there is important contamination of hair directly from the air. In occupational exposure, blood and urinary mercury are also good choices, for relatively short-term exposure.

External contamination of hair is a very important source of error in in most hair (or nail) metal analyses.

For lead and inorganic lead compounds, the best indicator is the lead concentration in blood:   reliable information is available on the relationship between blood lead and the critical effects. For tetraethyl lead (octane enhancer in gasoline) the best indicator is urine: this form of lead is excrted in the urine.

For cadmium, the best indicator is cadmium in urine: again, very good information is available on the urinary cadmium concentrations and the critical effects.

For most other metals, analysis of biological specimens for the assessment of exposure is less reliable, and for most, the best "tissue" is urine (nickel, chromium, aluminium, arsenic, cobalt, selenium)  

It depends on the habitat you want to monitorize... In aquatic or marine environments, chick feathers of birds are one of the most commonly used biological sample to monitorize heavy metal levels.

I believe you are interested in human monitoring ?  In which case hair is the best non-invasive tissue to sample but it can be difficult to relate to level of exposure.  Invasive samples could include liver, bone etc. depending on the metal of interest.

Regards

Paul Jones

Surely hair and nails are the best non invasive bioindicators of metals exposure, especially for mercury and methylmercury there is a wide bibliography about it.

All the best

Stefania

I hope you will find useful suggestions in these publications.

beste regards

stefania

Article Trace elements in scalp hair of children living in differing...

Article Children exposure to trace levels of heavy metals at the nor...

Article Mercury analysis in hair: Comparability and quality assessme...

Public health laboratories in the US tend to use blood or urine to determine exposure to metals, depending on the toxicological mechanism. Here is a blog full of resources related to human biomonitoring and public health laboratories: http://www.aphlblog.org/2014/10/biomonitoring-public-health-laboratory/

Choosing a suitable method should also consider the chemical and biochemical properties of a target pollutant. Different pollutant can be found deposited in different organs due to many factors.

The most reliable species are sesil forms such as mussels, gastropods, and some invertebratas.

Hair and nails is a common biological sample but there are some of the study indicate that hair sample is not reliable except detecting mercury. The selection of the sample also rely of the properties of the element and route of exposure.  

Source: Commercial Hair Analysis: A Cardinal Sign of Quackery by Stephen Barrett, M.D.

Since you are considering trace metals in human biological samples, I suggest you use hair, nails or urine. However, I want to strongly suggest that you dump the term "heavy" for "trace" since you will be working on biological samples.

Long-term exposure and short-term exposure are important since you are considering trace metals in human biological samples. While the data on total body burden from exposure to trace elements are not yet available, a whole blood sample is the best for measuring levels and determining the recent exposure (e.g., Pb, up to 35 days). However, the blood level does not often reflect whole-body mineral content. Therefore, hair is an appropriate alternative to blood and is ideal for measuring accumulated toxic metals over a period of time (e.g., Pb, up to 125 days). Urine and fecal testing reflect the levels of heavy metals excreted from the tissues and the body, which are not recommended unless taking a chelating agent that helps extracts' metals in urine and can be used to evaluate the efficacy of the treatment.   

Hair and  and nails are the best non invasive bio-indicators of metals exposure.I strongly recommend hair sample.

I agree with the use of hair, nails and urine are the best invasive bio-indicator of trace metal, however I believe it does reflect the excretion dose. What about the dose of absorption and storage of trace metals in the human body?

i recommend skin as it has large surface area - external contact and

for internal absorption of heavy metals blood can be used.

There is no generic answer to your question: the best sample depends on the exposure pattern and on the metal of interest - and also of the time-span of the exposure and of interest.

Hair is be appropriate for mercury exposure, when the species of interest is methyl mercury (= dietary exposure mainly from fish), and the interest is long term exposure or changes of exposure with time (analysis of different parts of hair) but it is not suitable for the assessment of exposure to metallic or inorganic mercury species especially at work place, where there is important contamination of hair directly from the air. In occupational exposure, blood and urinary mercury are also good choices, for relatively short-term exposure.

External contamination of hair is a very important source of error in in most hair (or nail) metal analyses.

For lead and inorganic lead compounds, the best indicator is the lead concentration in blood:   reliable information is available on the relationship between blood lead and the critical effects. For tetraethyl lead (octane enhancer in gasoline) the best indicator is urine: this form of lead is excrted in the urine.

For cadmium, the best indicator is cadmium in urine: again, very good information is available on the urinary cadmium concentrations and the critical effects.

For most other metals, analysis of biological specimens for the assessment of exposure is less reliable, and for most, the best "tissue" is urine (nickel, chromium, aluminium, arsenic, cobalt, selenium)  

Generally, in looking for accuracy in measuring heavy metal exposure, I would recommend urine samples first, and hair samples next. However I also agree that blood samples for lead are usually the best measure for testing exposure to this metal.

What about the dose of absorption and storage of trace metals in the human body?

Lead is taken up both by inhalation and by ingestion.Generally, about 30-40% is taken up from the lungs into the circulation. Lead accumulates in the skeleton. Skeleton usually contain about 40% of the body's lead content.Half-life of lead deposited in bone is 10-20 years ("the slowly exchangeable fraction"). the rapidly exchangeable fraction represents approximately 10% of the body lead content, and has a half life of about 30days. This fraction consists of lead in soft tissue, which is in direct contact with lead in blod.Blyet rinsed mainly via the kidneys, but also a little bile.Especially for organic lead applies that these drugs are absorbed by the skin, and that organic lead crosses the blood-brain barrier more readily than inorganic lead.

Heavy metals are taken into the body via inhalation, ingestion, and skin absorption. If heavy metals enter and accumulate in body tissue faster than the body’s detoxification pathways can dispose of them, a gradual buildup of these toxins will occur.

I agree with the suggestion of above mentioned researchers opinion. It also depends upon the type of metals and area of contact with the organism.

Thats true Muhammad, I forgot to include that point. Thanks for the good point addation.

Hair, here there and everywhere! It's all we've got so far!