I am a researcher in academic lab and offen work with radioatcive
materials, such as P32, H3, etc to trace DNA, RNA or proteins. While
handling the radioactive material, we try our best to take precaution,
but of course there are always some residual radioactive activity left
around the lab (within the acceptable safety range).  My question is,
how is the effect of radioactive activity on human? What is the
consequence of being exposed with moderate level of radioactive
(within the lab safety range, according to lab safety guideline)? 
Also, what is the effect for pregnant woman who works in this type of
environment? How will it affect the fetus?

Hi travelmug-ga, and thanks for your question.  My congratulations if
you are pregnant.

One of the best summary discussions of radiation exposure to pregnant
or non-pregnant individuals can be found in the 287 page Department of
Energy Radiological Worker Training manual, which you can download
here:
http://www.eh.doe.gov/techstds/standard/hdbk1130/hdbk1130cn2.pdf

On page 30, the authors summarize what you probably already know about
the basics of sensitivity to radiation:

"In general, the human body becomes relatively less sensitive to
ionizing radiation with increasing age.  The exception is that elderly
people are more sensitive than middle-aged adults due to the inability
to repair damage as quickly (less efficient cell repair mechanisms)."

On page 31, Section D, there is a great summary of what is currently
known about radiation exposure to the fetus, based on data from atomic
bomb survivors in Japan:

"Although no effects were seen in Japanese children conceived after
the atomic bomb, there were effects seen in some children who were in
the womb when exposed to the atomic bomb radiation at Hiroshima and
Nagasaki. Some of these children were born with a slightly smaller
head size, lower average birth weight, and increased incidence of
mental retardation. Some later showed lower IQ test scores and slower
scholastic development, smaller physical size, and increased incidence
of behavioral problems."

It should be noted, however, that the effects above are based on
higher doses that those found in research labs.  I include the
information to give you some idea of what happens at higher, but not
lethal, doses.
____________

The fetus is not equally susceptible to radiation exposure throughout
the course of pregnancy:

"The embryo/fetus is most susceptible to developing adverse health
effects if exposed during the time period of 8 - 15 weeks after
conception."


On page 39 of this report, the authors give guidelines for radiation
exposure limits to the pregnant radiation worker:

"b. DOE limit
For a declared pregnant worker who continues working as a radiological
worker, the following
radiation dose limit will apply.
1) The dose equivalent limit for the embryo/fetus (during the entire
gestation period) is 500
mrem.
a) Measures must be taken to avoid substantial variation above the uniform
exposure rate necessary to meet the 500 mrem limit for the gestation period.
b) The DOE RCS recommends that efforts be made to avoid exceeding 50
mrem/month to the embryo/fetus of the declared pregnant worker.
2) If the dose equivalent to the embryo/fetus is determined to have
already exceeded 500 mrem when a worker notifies her employer of her
pregnancy, the worker shall not be assigned to tasks where additional
occupational radiation exposure is likely during the remainder of the
pregnancy."

This means that if you are pregnant, you should inform the radiation
safety office, preferably in writing, that you are pregnant and the
estimated date of conception.  Otherwise, they do not know to apply
the lower exposure limits when analysing your radiation badge.  The
exposure limit for pregnant women is about 10% of the limit for
non-pregnant workers (5 rem/yr), so even if your lab is "within
guidelines," you may still be close to your limit, depending on how
much work you're doing with isotopes.

____________

There are, of course, many other hazards in the typical research lab,
including solvents, chemicals, etc.  For more information on
non-radiation hazards, I suggest this recent article from the Journal
of Occupational Health:

Kumar S. Occupational Exposure Associated with Reproductive
Dysfunction. J Occup Health 2004: 46; 1-19.

http://joh.med.uoeh-u.ac.jp/pdf/E46/E46_1_01.pdf
____________

Also of interest, WPI has a good description of individual isotopes
and methods to improve shielding and minimize exposure.  You can read
more here:
https://www.wpi.edu/Admin/Safety/RSO/Training/trm.html#A7

Some isotopes, particularly those used in biological research labs,
have the potential for ingestion or inhalation.  The WPI report above
gives annual limits on intake (ALI) for various isotopes in Table 2
about 3/4 of the way down the page.
____________

In the Army report, described below, we also have a discussion of in
utero effects in Chapter 9, page 221-227.  In these pages is a wealth
of information on effects of radiation exposure in utero at every
stage, along with discussions of who has analysed the data and the
various ways of interpreting it.
http://www.afrri.usuhs.mil/www/outreach/pdf/tmm/chapter9/chapter9.pdf

On pg. 227, the authors summarize the current recommendations and
predicted effects:

"Current estimates predict two to three leukemia deaths for each
10,000 children receiving 1 Gy of low-LET [Linear Energy Transfer]
radiation in utero. Solid tumors will account for an additional
2.0-2.8 deaths in the same 10,000 children. The combined increased
mortality from childhood cancer as a result of in utero exposure is
4.0-5.8 per 10,000 children per Gy. The natural total risk of
mortality from malignancy through age 10 is one in 1,200. If an
average chest X ray delivers 250 mGy to the fetus, the probability of
that fetus developing a fatal cancer during childhood is one in a
million.  The NCRP recommends that fetal exposure be limited to 0.5
mSv (0.05 rem) per total gestation period or 0.05 Sv/month.244 The
increased risk for mortality in children receiving the limit of 0.05
Sv/month in a single exposure would be two to three per 100,000
children."

==================================

In terms of your own exposure and possible effects, the same DOE
report cited above discusses the typical descrease in life expectancy
based on exposures at or below the regulation limits (see Table 2-2 on
page 33):

"Health Risk Estimated Loss of Life Expectancy

Smoking 20 cigarettes a day        6 years
Overweight (by 15%)                2 years
Alcohol consumption (U.S. average) 1 year
Agricultural accidents           320 days
Construction accidents           227 days
Auto accidents                   207 days
Home accidents                    74 days
Occupational radiation dose
  (1 rem/y), from age 18-65
  (47 rem total)                  51 days
All natural hazards
  (earthquakes, lightning, flood)  7 days
Medical radiation                  6 days"


On page 32, the authors put the risks another way, comparing to the
overall risk of cancer in the US population:

"The risk of cancer induction from radiation exposure can be put into
perspective. This can be done by comparing it to the normal rate of
cancer death in today?s society. The current rate of cancer death
among Americans is about 20 percent. Taken from a personal
perspective, each of us has about 20 chances in 100 of dying of
cancer. A radiological worker who receives 25,000 mrem over a working
life increases his/her risk of cancer by 1 percent, or has about 21
chances in 100 of dying of cancer. A 25,000 mrem dose is a fairly
large dose over the course of a working lifetime. The average annual
dose to DOE workers is less than 100 mrem, which leads to a working
lifetime dose (40 years assumed) of no more than approximately 4,000
mrem."

Their radiation exposure limits are summarized in Table 3-7 on page 37.

As the authors state, only you can decide what is acceptable, based on
good information:

"b. Acceptance of a risk:
1) is a personal matter.
2) requires a good deal of informed judgment."

The DOE report also gives us some idea of how the scientific community
views the guidelines they have given:

"The risks associated with occupational radiation doses are generally
considered acceptable as compared to other occupational risks by most
scientific groups who have studied them. There are some scientific
groups who claim that the risk is too high. DOE continues to fund and
review worker health studies to address these concerns."

____________

On page 195 of the Army report below, we get some other radiation
exposures to get some scale on lab exposures and the limits discussed
above:
http://www.afrri.usuhs.mil/www/outreach/pdf/tmm/chapter9/chapter9.pdf

"A cross-country airplane flight increases individual exposure by 0.2
mrem/hour because the level of cosmic radiation is greater at 36,000
feet than at sea level.[7]"

"Spaceflight increases exposure to solar and cosmic radiations; Apollo
astronauts traveling to the moon received an average of 275 mrem over
19.5 days.[8]"

"Radiation is also emitted from consumer products, such as color
television sets (averaging 0.3-1.0 rem/hour of use), video terminals,
smoke detectors (which contain an alpha emitter, usually
americium-241), and dinnerware that uses uranium for an orange
color.[7,15] Ophthalmic glass, used in prescription lenses, contains
trace impurities of thorium-232, and uranium is added to dental
porcelain to give dentures a natural fluorescent quality.[15] The
latter may result in an alpha radiation dose of 60 rem/year to the
gums.[15]"

______

Basically, there is no threshold for possible radiation effects,
however pseudo-thresholds have been estimated for some types of
cancers:

"Mathematical models predicting cancer risks based on observations
from high radiation exposures imply that 120-180 additional cancer
deaths will occur for every million persons receiving 1 cGy of
radiation. This estimate range includes the incidence of all cancers
and presumes that no thresholds for induction exist. Some evidence
indicates that thresholds for radiation-induced cancer do exist,
ranging from 0.01 Gy for breast cancer to 0.2 Gy for leukemia. [1cGy =
1 rem]"

For more on low level radiation exposure, I would take a look at the
bleakly named text
Textbook of Military Medicine: Part I: Warfare, Weaponry, and the
Casualty, Volume 2, Medical Consequences of Nuclear Warfare.
http://www.afrri.usuhs.mil/www/outreach/pdf/tmm/frontmatter.pdf

____________

Another way to look at risk is simply the likelihood of a genetic
mutation with a given dose.  The Army report above describes this as
the genetically significant dose (GSD):

"The genetically significant dose (GSD) is the dose of ionizing
radiation to the gonads that may  result in increased incidence of
genetic mutations in germ cells.[7,8] Estimation of the GSD  takes
into account the number of persons of reproductive age in a particular
group in determining a collective dose. In the United States, the GSD
from background and generated radiation sources is 122 mrem per person
(Table 9-7).[7] The GSD from occupational exposure in the military
service is less than 0.04 mrem per person, which is less than that
received in a national research laboratory (< 0.2 mrem/year) or a
nuclear power plant (< 0.15 mrem/year). Most occupational exposures
are less than those received from consumer products over the same
period."

========================

I hope this information is useful.  I wish you the best with your
pregnancy and research.  Please feel free to request any clarification
prior to rating.

          -welte-ga

Very detailed answer! I'm very satisfied with the answer and this is
exactly what I am looking for. Thank you very much!