Project Leader and Principal UC
Robert K. Webster, professor,
Department of Plant Pathology, University of California, Davis
|Rice cultivation is associated with exposure to numerous
agents that may affect respiratory health, including dust from field preparation and
harvesting, dust mites, molds and other organic antigens, pesticides, as well as smoke
from burning rice stubble. In view of these concerns the Rice Research Board commissioned
a study to characterize the respiratory health of California rice farmers.
the spring of 1991 a team of medical researchers from the UC Davis Division of
Occupational and Environmental Medicine examined nearly 500 volunteer rice farmers in an
attempt to gauge their respiratory health. The more significant findings and results of
the study are summarized at right.
- 475 rice farmers from eight California rice growing counties participated in the study.
These included 464 men and 11 women. Because of the small number of women, subsequent
analysis was restricted to men.
- The mean age of the participants was 48.3 years, and mean duration of rice farming was
25.7 years. The median acreage farmed was 340 acres. Approximately 90 percent of the
participants reported personal involvement in cultivation activities such as field
preparation, harvesting and post-harvest burning of rice stubble.
- Smoking was uncommon in this group. Approximately 9 percent were current smokers,
compared with approximately 25 percent among California's male population.
- Prevalence of symptoms commonly related to smoking, such as chronic cough and chronic
phlegm production, were lower than seen in the general population. No association was
found between these symptoms and farming tasks or exposures.
- There was an increased prevalence of several allergically related conditions, including
physician-diagnosed asthma (7.1 percent of participants), persistent wheeze (8.8 percent
of participants), and hayfever (25.7 percent of respondents). Physician-diagnosed asthma
was associated with field preparation activities and persons with greater than 35 years
involvement in such activities had twice the prevalence as did persons with fewer than 12
years involvement. However, researchers caution that while the association between the two
is "biologically plausible," further studies would help clarify whether a
cause-and-effect relationship exists.
- Pulmonary function results for the subjects as a group were at or above population norms
for the major respiratory indices. Physicians noted reduced mid-expiratory flow rates,
which raises the possibility of an occupational role in allergically related respiratory
conditions in the group. No association was found between various occupational exposures
and pulmonary function. As expected, pulmonary function was reduced among current smokers.
- Chest X-rays were performed on 464 individuals; all subjects received copies of their
results several weeks following the screening. In addition, 179 chest X-rays were
evaluated by specially trained health professionals. Of this latter group, 19 showed
increased radiologic lung markings. Four showed abnormalities in the thin outer membrane
lining of the lung. Both of these findings are suggestive of dust-related lung conditions
and are increased over the frequency expected in the general population.
So what do these results bode for rice farmers? As a group, rice farmers don't smoke as
much as the general population and therefore show fewer smoking-related symptoms. Furthermore,
rice farmers show normal or above-normal pulmonary function.
However, an increase in physician-diagnosed asthma, hayfever and reduced mid-expiratory
flow rates raise the possibility of an occupational role in allergically related
respiratory conditions, such as asthma, in the group. An increased prevalence of chest
X-ray findings consistent with dust or fiber exposure was also found. Apart from a
suggestive association between field-preparation activities and physician-diagnosed asthma
no clear associations exist between respiratory health and occupational exposure.
In interpreting these results, however, several points must be emphasized. First, the
study involved persons currently or very recently active in rice farming. Persons with
health problems may be less likely to remain rice farmers or to participate in a study.
Thus, the study population may have been healthier than a group followed through time from
the beginning of a farming career. Secondly, exposure assessment was based on
questionnaire response, rather than actual measured exposure levels. Thus, different
exposure levels could not be measured. Thirdly, the study looked at farmers only at a
single point in time, with no prior "baseline" data with which to compare. A
subject with above average respiratory function could conceivably have his respiratory
function impaired yet still test in the "normal" range. Finally, there was no
non-farming control population for comparison of smoking rates, exposures and prevalence
of health outcomes.
On the basis of the data collected, the researchers made the following recommendations:
- Take protective measures to preserve respiratory health. In addition to refraining from
smoking, use of enclosed-cab farm machinery and personal respiratory protective equipment
to minimize dust exposure should be utilized.
- Direct measurement of exposures associated with various farming tasks should be
undertaken. This would involve area and personal sampling measurement for ambient dust
levels for the various tasks required in rice cultivation. In addition, the dust and
airborne particles from various Wks associated with rice cultivation, including burning,
should be analyzed for crystalline silica, amorphous silica, fibrous and non-fibrous
silicates and biological activity.
- Prospective evaluation of respiratory health, including before and after measurements of
breathing function for specific tasks, across a season, and over a period of years should
be undertaken to clarify the nature of respiratory health effects and whether they are
associated with specific activities. This study suggests two yeas in particular that
should be further investigated: occupational causes of increased pulmonary markings
suggestive of dust exposure and occupational causes of airway hyper-responsiveness or