On Thu, 6 Jul 2000, Carla Frenchko wrote:

> Our PA asked a PA student what a combination of
> bleach and formalin would have on tissue
> histologically. I have never come across this question
> before and am wondering if anyone has an answer.

This question has resulted in a variety of replies to the listserver,
including some that warned of dangerous reactions products from bleach
(sodium hypochlorite dissolved in water) and formalin (formaldehyde
dissolved in water; the actual product in solution is methylene
hydrate, HOCH2OH, not formaldehyde, CH2O, which is a gas. In strong
formalin the methylene hydrate is present as low polymers, which
depolymerize on dilution.).

According to the 3rd edn of J. Frederic Walker's book "Formaldehyde"
(Litton Educational Pub., 1964; reprinted by Krieger, 1975) gaseous
chlorine reacts with paraformaldehyde, a high polymer of formaldehyde,
to form either phosgene, COCl2, (in direct sunlight) or carbon monoxide,
CO, (in ordinary light). The other product of both reactions is
hydrogen chloride (HCl) and some heat is needed to start these
reactions. Phosgene and CO are both poisonous gases. No reactions with
hypochlorite are described in the book, but bleach is used as a
reservoir of available chlorine, and it is reasonable to suspect that
mixing bleach with formalin would generate some carbon monoxide and
perhaps some phosgene if done in bright light. No product useful to
the histologist is formed by mixing bleach with formalin, so this is
not a good thing to do.

The various chloromethyl ethers, mentioned in some replies, are formed
when formaldehyde and methyl alcohol (which is present in formalin)
react with high concentrations of hydrogen chloride. This might happen
in the atmosphere above a sink into which someone poured concentrated
hydrochloric acid and formalin. The high concentrations of reactants
could coexist in droplets in the fumes. Some chloromethyl ethers are
carcinogenic (this from our Safety people, not from Walker), so it's
bad to pour conc HCl and formalin (any strength) into a place where
their vapours could mingle. If you do this, don't inhale the products. 

The chloromethyl ethers react rapidly with water, generating formaldehyde,
chloride and hydrogen ions. The possible carcinogenic hazard doesn't 
exist when formalin or paraformaldehyde is mixed with dilute hydrochloric
acid (as in some decalcifiers) or with water containing chloride ions (as
in formal-saline).  

To return to the original question:  What kind of person asked this of
what kind of student, and why?  Some reactions of hypochlorite with
tissue components can be found in the histochemistry texts used by
research workers (Pearse, Lillie, Ganter et Jolles etc), and the
reactions of formaldehyde with proteins are described in most textbooks
of histological technique or histochemistry published since 1955 and
in a few older works. There is no short answer to the question. 

Hypochlorite will cause oxidative deamination. Formaldehyde adds to
amino groups and cross-links some of them, if they are already close.
Both hypochlorite and formaldehyde have other reactions, with proteins
and lipids. A previously soluble protein (gelatin) exposed to alkaline
hypochlorite becomes insoluble in water; the reason has not (to my
knowledge) been found.
 
If PA and PA student translate to PhD and PhD candidate, this question
might generate half an hour of interesting discussion a PhD oral exam.

If you can explain a "PA" and his/her educational background, and that
of her/his student, it will be easier to come up with an answer to this
silly question. This enquiry carries a strong impression that a bunch of
eager students is being trained by some inadequately educated elder person.
Sorry for such a convoluted pair of sentences, but I don't want to be
accused of attributing ignorance and/or incompetence to any unknown
member of either of the two sexes. ("What! Only 2! Go out and order
some more." -- Spike Milligan, ?1952).

Bottom line 1. Don't mix formalin with bleach.
Bottom line 2. If asked a seemingly ridiculous question: Ask why.

 John A. Kiernan,
 Department of Anatomy & Cell Biology,
 The University of Western Ontario,
 LONDON,  Canada  N6A 5C1