Hi…
Out comes my wellness side. I have a high awareness of what is in
our environment.
There are a lot of things that we use that rarely causes people
problems. However, a lot of what is used to clean (or to make suds,
etc.) do create some issues.
Below is the info…anything listed with toxnet has some sort of
toxicity. That said, jewelers are around much more toxic compounds.
If you worry, check there. About anything, really. Then make a
decision based on not heresay, rumors, or
misunderstandings.
I don’t recommend borax being heated without a mask on for people
with any allergies. I’ve developed allergies that way (chlorine in a
hot tub…now I’m allergic to chlorine and cannot take a shower or
use plain water unless it’s specially filtered). But as a flux, borax
would probably be okay because of the amounts. (don’t use if you have
allergies or lung issues of any kind.)
I only had a quick glance, but nothing said it was tetragenic (sp).
Then again…that just means it was tested / proven on pregnant
women. Some things are assumed non-tetragenic because they haven’t
been proven to be (that means they still could be, so be aware).
Anything that goes into your body affects it…from any orifice and
any part of your skin (more prolonged exposure). If you don’t believe
me, take one shoe and sock off and place a slice of onion on your
foot. You’ll taste it in a short time.
That said, I still use chemicals in this work, I rarely wear a
breathing mask…it only works if you use it until you leave the room
or the room air is circulated out the door, but I try to minimize the
risk of exposure based on info I find at toxnet and additional
research.
None of what I say should be construed as medical advice. I am not a
doctor, nor do I play one on tv. 
The more relevant bits from toxnet are first in the toxnet info
below.
Kim
HUMAN EXPOSURE STUDIES The incidence of respiratory symptoms,
pulmonary function, and abnormalities of chest radiographs to
estimated exposures of borax dust was studied in 629 actively
employed borax workers. The subjects, who had worked in this
field for at least 5 years, were interviewed to obtain
on persistent respiratory symptoms and smoking
habits. Occupational histories were reviewed to assess
occupational exposure, and pulmonary function tests and chest
radiographs were performed. Estimated exposure ranged from 1.1 to
14.6 mg/cu m. Symptoms of acute respiratory irritation such as
dryness of the mouth, nose, or throat, dry cough, nose bleeds,
sore throat, productive cough, shortness of breath, and chest
tightness were related to exposure of 4.0 mg/cu m or more.
Symptoms of persistent respiratory irritation meeting the
definition of chronic simple bronchitis were related to exposure
among nonsmokers. Radiographic abnormalities were uncommon and
were not related to dust exposure. /Data suggests/ that borax
dust appears to act as a simple respiratory irritant. [Garabrant
DH et al; British J Ind Med 42 (12): 831-37 (1985) ]
/SIGNS AND SYMPTOMS/ Workers industrially exposed to borax often
suffer from chronic eczema. Long-term exposure to borax dust may
lead to inflammations of the mucous membranes of the airways
(bronchitis, laryngitis) and to conjunctivitis. [International
Labour Office. Encyclopedia of Occupational Health and Safety.
Vols. I&II. Geneva, Switzerland: International Labour Office,
1983., p. 320]
/OTHER TOXICITY INFORMATION/ Fatal doses for humans are
variously estimated to be 5 to 6 g for children and 10 to 25 g
for adults. [Clayton, G. D. and F. E. Clayton (eds.). Patty's
Industrial Hygiene and Toxicology: Volume 2A, 2B, 2C: Toxicology.
3rd ed. New York: John Wiley Sons, 1981-1982., p. 3059]
/OTHER TOXICITY INFORMATION/ Borax and boric acid used in
powders and ointments have resulted in serious poisonings and
death. [Seiler, H.G., H. Sigel and A. Sigel (eds.). Handbook on
the Toxicity of Inorganic Compounds. New York, NY: Marcel Dekker,
Inc. 1988., p. 135]
Skin, Eye and Respiratory Irritations:
... Borax... /is an irritant/... when in contact with skin and
mucous membranes of the eyes, nose and other sites in the
respiratory tract.... [American Conference of Governmental
Industrial Hygienists. Documentation of Threshold Limit Values
for Chemical Substances and Physical Agents and Biological
Exposure Indices for 2001. Cincinnati, OH. 2001., p. 1]
... May produce irritation of the nasal mucous membranes, the
respiratory tract, and eyes. /Boron compounds/ [Sittig, M.
Handbook of Toxic and Hazardous Chemicals and Carcinogens, 1985.
2nd ed. Park Ridge, NJ: Noyes Data Corporation, 1985., p.
138]
/CASE REPORTS/ Occupational exposure for 6 years to a soap
powder containing 78.6% sodium borate resulted in hair-loss in
one man. The patient was advised to avoid all contact with the
soap powder, and subsequently, hair loss "subsided" and hair
growth returned "more or less" to normal. [Christian M, ed; J
American College of Toxicology 2 (7): 87-125 (1983) ]
/SIGNS AND SYMPTOMS/ Ingestion of 5 to 10 g by young children
can cause severe vomiting, diarrhea, shock and death. [The Merck
Index. 10th ed. Rahway, New Jersey: Merck Co., Inc., 1983., p.
1231]
/SIGNS AND SYMPTOMS/ /Effects from/ ingestion /include/
abdominal pain, diarrhea, headache, nausea, vomiting, weakness,
convulsions. [IPCS, CEC; International Chemical Safety Card on
Borax. (April 1997). Available from
http://www.inchem.org/documents/icsc/icsc/eics0567.htm as of May
18, 2005. ]
(Okay, this is a WEIRD one, and I didn’t feel like checking to see
if they realized in the study that honey should NOT be given to
infants, and don’t know WHY borax was given…but it’s there.)
/CASE REPORTS/ A case-series report of seven infants (aged 6-16
weeks) who used pacifiers coated with a borax and honey mixture
for 4-10 weeks concluded that exposures ranged from 12 to 90 g,
with a very crudely estimated average daily ingestion of 18-56
mg of boron per kg of body weight. Toxicity was manifested by
generalized or alternating focal seizure disorders,
irritability, and gastrointestinal disturbances. Although infants
appear to be more sensitive than adults to boron compounds,
lethal doses are not well documented in the literature. [WHO;
Boron in Drinking-water: Background document for development of
WHO Guidelines for Drinking-water Quality (WHO/SDE/WSH/03.04/54).
Available from
http://www.who.int/water_sanitation_health/dwq/boron.pdf as of
May 9, 2005. ]