1999
Why did we do the study?
Earlier studies of animals and humans suggested that exposure to dioxin may cause health problems. Some of the health outcomes we wanted to study in humans involved the liver, the nervous system, the lungs, heart, the endocrine system, and the reproductive system.
What did the study find?
The results we report here do not describe an individual's personal risk for illness or disease. Instead, the study compared the health of dioxin-exposed workers as a group to the health of the non-exposed workers as a group.
We found more dioxin in the blood of the workers exposed to dioxin than in the blood of non-exposed workers. We wanted to see if the amount of dioxin in the blood was related to any potential adverse health outcome.
Our study did not always see a difference between the exposed and the non-exposed groups for every health outcome we tested. In fact, for most of the outcomes, the health status of exposed and non-exposed workers were basically the same.
When we compared the two groups, we found small differences for the following:
- gamma glutamyl transferase (GGT) levéis. GGT is a liver enzyme
- Male reproductive hormones levels
- Lipid levels: high density lipoprotein (HDL) and triglycerides
What about other outcomes?
When we compared the exposed group to the non-exposed group, we found no differences between the two groups for:
- chronic bronchitis
- chronic obstructive pulmonary disease (COPD)
- diabetes mellitus
- digestive function
- intestinal disease
- heart disease
- high blood pressure
- immune system function
- liver disease
- lung function
- lung disease
- mood and depression
- peripheral nervous system function
- porphyria cutanea tarda ((PCX), a liver disease)
- thyroid disease
- total cholesterol
Next, we give you more details about the differences we found.
Blood-Dioxin
During the medical exam, we collected blood samples to determine the level of dioxin in the blood. Blood levels of dioxin in U.S. residents are generally less than 20 ppt (parts of dioxin per trillion parts of lipid). The average level is about 7 ppt. In this study, we found the average dioxin level in the non-exposed group was 6 ppt. The highest level in the non exposed group was 20 ppt.
The highest blood-dioxin level among the exposed workers was 3,400 ppt. The average blood-dioxin level for the exposed workers was 220 ppt. This is about 11 times higher than the highest value in the non-exposed group.
Typically, the longer a worker was exposed to dioxin the more dioxin was present in the blood.
More recent studies show that blood-dioxin levels of exposed persons are decreasing slowly over time. We expect that after every 7 to 10 years about one-half the dioxin is removed from the body.
Now we will discuss the biological indicators of health that were related to increases in blood-dioxin levels.
Liver
Our study looked at 10 different lab tests which might show liver effects. Using these tests, we did not find a direct link between dioxin exposure and liver disease. However, dioxin-exposed workers who drank more than 2 alcoholic drinks per day over their lifetime were more likely to have GOT levels above the upper limit of normal, 96 IU/L. GOT (gamma glutamyl transferase) is one of the 10 tests we looked at for liver effects. It is an enzyme produced by the liver. GGT helps the liver to work properly. For persons with no dioxin exposure, increased GGT may also indicate excessive alcohol use.
We looked at dioxin-exposed workers who drank alcohol. As both their alcohol use and their blood-dioxin levels increased, the more likely their GGT was to be increased. The increased GGT levels in the exposed group may have been caused by the dioxin working together with the alcohol. The higher GGT levels are not necessarily cause for concern because the other 9 tests for possible liver effects showed no differences between the exposed and the non-exposed. The high levels noted in this study do not appear to be related to liver disease.
Male Reproductive (Sex) Hormones
The male reproductive system uses several hormones to function properly. These hormones are chemicals produced in the body to regulate reproductive activity. Three of the hormones we studied were:
- testosterone
- follicle stimulating hormone (FSH)
- luteinizing hormone (LH)
Because of the few females in this study, we did not study the relationship between dioxin exposure and female reproductive hormones.
Testosterone is necessary for maintaining sex drive in males, and to have sex. Testosterone also controls the number and quality of sperm produced. A wide range of testosterone levels is considered normal.
We divided the dioxin-exposed workers into 4 almost equal groups (or quarters) based on blood-dioxin levels. The lowest exposed group had levels that were under 20 ppt. This is similar to the levels seen in the non-exposed group.
The 2nd quarter had levels between 20-75 ppt. The 3rd quarter had levels between 76-240 ppt. The highest quarter had levels between 241-3,400 ppt.
This study showed that males in the exposed group, with blood-dioxin levels greater than 20 ppt, had lower average testosterone levels than those in the non-exposed group. Testosterone levels less than 10.4 nmol/1 were considered low. However, the average testosterone levels in each of the 4 exposed groups were still within the normal range.
The NIOSH study also showed that the exposed group had higher FSH and LH levels. FSH promotes the development of sperm in males. LH acts along with FSH to stimulate development and function of Leydig cells which form part of the tissue in the testes. FSH levels above 31 IU/L were considered high. LH levels above 28 IU/L were considered high.
In workers with blood-dioxin levels greater than 241 ppt, more of the workers (11%) had higher than normal levels of FSH than in the non exposed group (6.1%). In those whose blood-dioxin levels were above 20 ppt, more (10.9%) had higher than normal LH levels than in the non exposed group (6.5%). However, increases in the FSH and LH hormones were not necessarily seen in the subjects with decreased testosterone levels.
Levels of testosterone, FSH, and LH vary a lot among males. Since the NIOSH study did not look at other indicators of reproductive health, such as sex-drive or sperm count, we cannot be sure if the hormone level differences seen in our study mean that dioxin-exposed men have more problems with the desire to have sex and fathering children than non-exposed men. However, based on the study results, we believe that it is unlikely that the reproductive health of the exposed men is impaired.
Lipid Levels
Lipids are stored in the body as a source of fuel. Lipids are basically fats. They are an important part of the cells in your body. They make up part of your blood cells. High lipid levels can increase the risk for heart disease. The NIOSH study and other medical studies of human exposure to dioxin have shown no significant relationship between exposure to dioxin and heart disease.
However, some studies that looked at causes of death among dioxin exposed workers compared to causes of death in the general U.S. population have shown an increased risk for heart disease. Further studies must be done to draw firm conclusions.
Our study measured total cholesterol, triglycerides, and high density lipoprotein (HDL) levels among exposed and non-exposed workers. A total cholesterol level equal to or above 240 mg/dl (6.21mmol/l) was considered high.
A triglyceride level equal to or above 250 mg/dl (2.82 mmol/1) was considered high.
HDL is also known as "good" cholesterol. This lipid level is also a marker for risk of heart disease. An HDL level equal to or below 35 mg/dl (0.91 mmol/1) was considered low. It is good to have an HDL level above 35 mg/dl.
We found that HDL levels tended to be lower in exposed workers whose blood-dioxin levels were in the highest quarter (241 - 3400 ppt) than in non-exposed workers.
A substantially increased triglyceride level and a substantially decreased HDL level may increase a person's risk for heart disease.
There are many factors that influence a person's triglyceride and HDL levels. Some factors are race, sex, body weight, smoking habits, and diabetes. Our study showed dioxin exposure was related to increased triglyceride and decreased HDL levels. However the influence of dioxin on these lipid levels was small compared to the influence of the other factors.
Despite the changes in triglyceride and HDL levels that were related to dioxin in this study, our study did not show an increased risk for heart disease among workers exposed to dioxin.
What do these results mean?
We cannot draw strong conclusions about health outcomes from just one study. When we look at the results of this study and other medical studies, we still do not have any strong evidence of a link between dioxin exposure and any of the health outcomes we studied.
One important point is that this NIOSH study includes one-time test results and medical history information collected more than 10 years ago. We cannot predict what similar tests might show if they were done today.
Another important point is that, overall, for most of the health outcomes we studied in 1987-1988, the health of workers exposed to dioxin was no different than the health of the workers not exposed to dioxin.
Would you like more information?
For more information or if you want a copy of the study reports, call: 1-800-356-4674 between 9:00 a.m. and 4:00 p.m., Monday through Friday, Eastern Time.