Acute Respiratory Effects of Exposure to Diesel Emissions in Coal Miners


SUMMARY

A study was conducted to determine if acute respiratory effects, measured in terms of changes in forced vital capacity (FVC), forced expiratory volume in one second (FEV,), and maximal expiratory flow rate at 50% of forced vital capacity (Vmax 50), were related to exposure to diesel emissions in coal miners. Sixty coal miners exposed to diesel emissions and 90 miners not exposed were tested before and after a work shift for ventilatory function changes. Significant work shift decrements in ventilatory function did occur in miners in both groups who smoked Cigarettes, but there were no significant differences in the ventilatory function changes between those miners exposed to diesel emissions and those not exposed either in the aggregate or under control by smoking status

Methods

Study design

A quasi-experimental study design was developed to examine the acute effects of occupational exposure to diesel emissions in white male underground coal miners during an 8-h work shift. An almost equal number of exposed and non­ exposed miners were drawn from both the day and the evening shifts to control the possibility of diurnal and circadian effects in ventilatory function.

All subjects were given ventilatory function tests before and after the work shift. For both groups, coal mine dust exposure and the 8-hr work shift were confounded independent vari­ables. The dependent variable, or focus of analysis, was the difference between the ventilatory function measurements ob­tained before and after the work shift.

Sample selection

The subjects in the exposed group were selected from a 6O-subject subsample drawn from a larger survey o f nearly 1,000 miners from 6 diesel-equipped mines located in Kentucky, Utah, Colorado, and Wyoming.

Diesel equipment at these mines were variously used in face hauling, in mucking operations, and for transporting both men and supplies. Approximately 10 persons were selected from each mine. 

Subjects were selected from those primarily involved in coal min- ing and carting, and then sampled randomly after stratification by work shift.

One departure from the sampling design did occur. A few subjects from one mine with diesel equipment were performing maintenance operations; hence, their exposure to diesel emissions may be confounded with other exposures to some extent.

A control group of 90 coal miners not exposed to diesel emissions was made up from two sources. Some were taken from existing NIOSH data bases. 

Others were taken at neighboring mines to diesel operations. Because time of year, calendar year, location of mine, time of shift, and race were all presumed to be important, general uniformity on these characteristics was achieved. 

Exposed and nonexposed miners were matched for geographic area, smoking status, and race, and matching on age and years of underground mining was attempted. This later matching was not optimal. 

Although differences in age and years of underground mining were generally related to pulmonary function, they were not related to acute pulmonary changes (see the section on Statistical Considerations).

The procedures and equipment used in examining the control group were similar to those used in the experimental group.

Ventilatory function

At least 5 FEV maneuvers were performed by each subject during each observation period on a dry rolling-seal spirometer Model 840; (Ohio Instruments, Madison, WI).

The flow and volume signals from the spirometer were recorded on FM analog tape (Model 3960; Hewlett-Packard, Waltham, MA) and were later analyzed by a digital computer (Model LSI-II; Digital Equipment Corp., Maynard, MA).

The FVC, FEV1 peak flow and flow rates at intervals of 5% of the FVC were measured and calculated from each flow-volume curve. The largest FVC, FEV1 and peak flow were used regardless of the curve or curves on which they occurred. The flow rates were obtained by aligning all of the flow-volume curves at TLC and using the largest observed flow at the volume of interest (maximal envelope flow-volume curves).

Decrements in flow rates during the work shift were also computed by aligning the before and after shift maximal envelope curves and obtaining the flow differences at the volume of interest.

Environmental sampling.

Each of the coal miners was asked to wear a personal dust sampler. This unit consisted of a NIOSH-approved pump, a nylon cyclone, and a cassette containing a polyvinyl filter.

The unit was operated at a flow rate of 2 L/min, at which rate particles were deposited on the filter with a cutoff point of IO Ilm (9).

In addition, each miner exposed to diesel emissions was asked to wear two passive dosimeters attached to the back of his helmet to measure concentrations of N02 (10).


Spot samples of diesel emissions contaminants were taken in areas where the miners worked to measure the amounts of carbon monoxide, carbon dioxide, oxides of nitrogen, nitrogen dioxide, sulfur dioxide, and formaldehyde.

The concentration of these contaminants was very low: in no case did they individually exceed 25070 of current standards or 40070 of current NIOSH recommendations (table 1). Time-weighted average measures would undoubtedly have been lower.

Other measurements. 

A detailed questionnaire including smoking inventory, demographic questions, work history, and respiratory symptoms was completed by each subject.

Cigarette smoking deserves special mention (11). Cigarette smoke shares many overlapping features with diesel exhaust such as particulate, NO2, CO, and formaldehyde, thus providing possible additivity of effects (12), although it should be noted that coal miners do not smoke while underground.

In addition, there was the potential for interaction between coal mine dust, diesel emissions, and cigarette smoking.

Smoking status distinguishing between current smokers, exsmokers, and nonsmokers was used in this analysis. All smokers were asked not to smoke immediately before or after their work shift in order to reduce the effect of smoking on ventilatory function.

Statistical considerations. 

One of the threats to the validity of the experimental analysis is that of differences between the control and experimental groups.
Several tests were performed to assess the degree of comparability between the experimental and control groups. The subjects in the control group were 15.1 yr older than those in the group exposed to diesel emissions, they were 1.8 cm shorter, had 15.9 more yr underground, and had 0.6 mg/m3 less exposure to respirable dust during the work shift, but they were more similar with respect to the percentage of current smokers and prevalence of persistent phlegm and simple coal workers' pneumoconiosis (table 2).

The question of whether or not these differences, or others, were likely to compromise comparison in this acute study was addressed using the technique of multiple regression analysis to test a set of potentially confounding variables for prediction of acute changes in ventilatory function.

These variables included age, height, years underground, persistent phlegm production, smoking status, and respirable dust exposure together with some of the more important first-order interactions including a diesel-by-dust interaction. 

Tests were run that combined both diesel exposure groups and separately by diesel exposure group. The only variable among these potentially confounding variables found to be statistically related to ventilatory function changes during the work shift was smoking status; hence, statistical control on smoking status was suggested.


Because neither age nor height was statistically related to ventilatory function or work shift changes, standardization of the measures of these variables was not necessary. Hence, all ventilatory function changes are presented as absolute value differences .

Results

In presenting the results from this study, only FVC, FEVl , and Vmax50 were reported, although peak flow and Vmax25 were also calculated. These three measures were selected as being reproducible measures of ventilatory function.

Work shift-coal mine dust effect.

There was an overall significant work shift-coal mine dust decline in FVC and FEV1 for both the exposed and the control groups (table 3). 

The average decline in FVC during the work shift for miners exposed to diesel emissions was 53 ml and for those not exposed it was 36 ml.

A similar change was seen in the FEV1 where there was a decrement of 55 ml in the group exposed to diesel emissions and a decrement of 34 ml in the nonexposed group. Nonsignificant changes in the same direction occurred for Vmax50 (-136 to - 65 ml/s).

Smoking status effect. 

Decrements in ventilatory function during the work shift were concentrated in the current smokers. All three measures of ventilatory function for current smokers showed significant decrements during the work shift, with the magnitude of the decrement increased for smokers over exsmokers and nonsmokers (table 3).

Smoker-nonsmoker contrasts equaled or exceeded work shift-coal coal mine dust effect changes for both the exposed and the nonexposed miners.

Diesel exposure effect. 

Two types of analyses were performed to test the effects of diesel emissions on the ventilatory function of coal miners.

An external analysis compared the before and after work shift ventilatory function changes in exposed and nonexposed miners. No statistically significant diesel exposure effect was seen using a t test for comparison between groups for any of the three measures of ventilatory function, either overall or with adjustment for smoking status (table 3).

This measure of diesel exposure combined the effects of diesel exhaust emissions, the interactive effect of diesel emissions, and coal mine dust aerosols, and, perhaps, other unmeasured differences between mines with diesel equipment and those without.

In order to further refine this comparison, controlling for measured potentially confounding effects, the diesel- nondiesel comparison was replicated by smoking status using covariance procedures (13) for simultaneous adjustment of the effects of age, height, years underground, and respirable dust exposure.

This covariance analysis also failed to find a significant difference between the exposed and nonexposed miners (table 4).

As a further line of argument beyond the diesel-nondiesel comparison, an internal analysis was performed in the miners exposed to diesel emissions to test for a dose-response relationship between ventilatory function changes during the work shift and N02 as the most potentially important diesel emissions exposure.

No significant relationship was found between concentrations of N02 and ventilatory function changes during the work shift. This lack of relationship persisted whether one considered all subjects together or looked at the relationship under control by smoking status. (These data are not shown.)

