American Journal of Industrial Medicine 1:449-452 (1980)

The Chemical Neutralization of Inhaled Sulfuric Acid Aerosol

Timothy Larson, PhD, Robert Frank, MD, David Covert, PhD, Dick Holub, MPH, and Michael Morgan, SCD

We hypothesize that gaseous ammonia (NH,) released into respiratory airways can neutralize inhaled acidic particles and alter or mitigate their toxicity. To test this hypothesis, we have examined the relationship between respiratory NH3 and the chemical neutralization of inhaled sulfuric acid (H2S04) aerosol in the surgically isolated upper airways of anesthetized dogs.

With air entering via the mouth, the NH3 concentration measured at the larynx, ranged between 30 and 225 parts per billion (ppb). The extent of neutralization at this point ranged from 8%-70% (n = 16) for the 0.5-pm particles and 7%-22% (n = 6) for the 1.0-pm particles. With air entering via the nose, the laryngeal NH3 concentration ranged between 50 and 220 ppb. Neutralization values were between 15%-65% (n = 8) for 0.5-pm particles, and between 16%-18% (n = 3) for 1.0-pm particles. For both routes of entry, the extent of neutralization was correlated with the laryngeal NH, concentration. We conclude that the chemistry of acidic particles is significantly altered by respiratory NH3 during inhalation. The extent of neutralization is related to both the NH, concentration in the airway and the size of the particle entering the airway. The extent of neutralization per ppb of laryngeal NH3 is related to the route of entry, being greater for the mouth than the nose.

Key words: sulfuric acid, respiratory ammonia, inhalation, chemical neutralization, upper airways

INTRODUCTION

Sulfuric acid (H2S04) and its neutralization products (NH4HS04, (NH4),SO4) with ammonia (NH,) are important chemical components of the submicrometer-diameter par- ticles suspended both in the ambient atmosphere and in occupational atmospheres [Charl- son et al, 1974; Brosset et al, 1974; Cunningham et al, 19761. We hypothesized that during inhalation acidic particles undergo further neutralization by respiratory NH, as they course the upper airways [Larson et al, 1977, 19781. To the extent that the acidity of these particles is irritating to the lung, this effect is important.

Departments of Environmental Health and civil Engineering, University of Washington, Seattle, Washington. Address reprint requests to Timothy Larson, Depts. of Environmental Health and Civil Engineering, University of Washington, Mail Stop FC-05, Seattle, WA 98195. Accepted for publication March 28, 198 I .

0271-3586/80/0103-04-0449 $01.50 0 1980 Alan R. Liss, Inc.

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MATERIALS AND METHODS

We examined the relationship between respiratory NH3 and the chemical neutral- ization of the inhaled aerosol in surgically isolated upper airways of anesthetized dogs. A total of nine dogs were studied. The dogs were anesthetized, tracheostomized, para- lyzed, and placed on a respiratory pump. The rectal temperature and the C 0 2 levels in the air expired from the lungs were monitored continuously. With the nose (or mouth) blocked, a vacuum was applied at the larynx and measurements were made on the air drawn through the mouth (or nose) and out the larynx at a flow rate of 0.1 literdsec. We found it necessary to lift the skin at the throat to reduce the flow restriction to acceptable levels (6p C 1 cm H20). The ammonia concentration in the air exiting the larynx was measured by a wet-chemical technique involving a gas-sensing electrode [Larson et al, 19791. The relative humidity of the air at this point was determined by measuring the dry bulb temperature and the dew point temperature (at 1/3 atmosphere) with a cooled mirror device. We also measured the air-flow rate and the pressure drop. The average time a parcel of air spent in the airway was measured in five dogs and found to be between 0.6-0.8 seconds.

The dog was then exposed to either 60 pg/m3 or 100 pg/m3 of a monodisperse H2S04 aerosol (ug s 1.2) at one of two different particle diameters, either 0.5 pm or 1.0 pm. This is about three times the H2S04 particle mass concentration found in the eastern United States during pollution episodes and represents the largest of the particles within the “accumulation mode” of the atmospheric aerosol (0.1 pm to around 1 pm in diameter). The extent of neutralization of the exposure aerosol was measured at the larynx.

The particles exit the larynx surrounded by gaseous NH3. This gas-particle mixture is immediately drawn through an NH3-absorbing device. The NH3 is removed from the mixture and the particles pass through. In this way, the reaction between the NH3 and the particles is stopped, and the chemical state of the particles exiting the absorbing device closely resembles the chemical state of those that exited the larynx. As the gaseous NH3 passes through the absorber, it diffuses to the absorber walls where it is removed (99 + percent efficient). In contrast, the particles do not diffuse rapidly, and pass through the tube where they are collected on filters for subsequent chemical analysis.

Before discussing the results of these measurements, let us briefly examine those factors that influence the extent of the neutralization of an acid particle. These include the relative amounts of NH3 and H2S04, the rate of reaction, and the time allowed for reaction. In our experiments, the most important of these factors is the rate of reaction. The factors that influence the reaction rate include the concentration of gaseous NH3, the H2S04 particle’s proportion of surface area to acidic mass, and the relative humidity. The higher the NH3 concentration, the more rapid the reaction rate. At a constant NH3 level, smaller particles are more rapidly neutralized than larger ones because smaller particles have more surface area exposed per mass of acid than larger ones. An increase in relative humidity increases the particle size because of the accretion of water. The particle’s surface area increases but its acidic mass does not. Therefore, increasing the relative humidity at a given NH3 concentration increases the reaction rate.

Neutralization of Inhaled Acid Aerosol 451

RESULTS We found a linear relationship between the percent neutralization and the NH3

concentration measured at the larynx. This is consistent with a rate of reaction limited by the rate of diffusion of gaseous NH, to the surface of each particle. We also observed large differences in both the NH3 concentrations and the extent of neutralization between different dogs but only small differences within each dog. Our measurements of human mouth NH3 levels show similar inter- and intra-subject variability.

Table I shows the range of neutralization values observed for both particle sizes (0.5 pm and 1.0 pm) and for both routes of entry (mouth vs nose). In all cases, these results reflect differences between the dogs. As expected, the extent of neutralization is less for the larger particles independent of the route of entry. However, at a given particle size, the range of observed values does not vary with the route of entry.

The mean laryngeal NH3 value was also independent of the route of entry (see Table 11). This result is consistent with our earlier assertion [Larson et al, 19771 that the NH3 concentration is determined by the last airway segment traversed by the air prior to sampling. In this case, the NH3 levels are determined by the larynx (and possibly by the oro-pharynx) and not by the more distant nose or mouth passages.

In contrast, the extent of neutralization is an integral property, reflecting the history of NH3 in all the airways traversed by the particle. Table 111 shows the percent neutral- ization per ppb of laryngeal NH3 for both particle sizes and routes of entry. As the

TABLE I. Range of Observed Neutralization Values (%)

d, = 0.5 pm d, = 1.0 km

Entry via mouth

Entry via nose

8-70 (No. = 16)

15-63 (No. = 8)

7-22 (No. = 6)

16-18 (No. = 3)

TABLE 11. Summary of Laryngeal NH3 Values

Entry via mouth Entry via nose

Mean (2 SE) Range

122 (k 14) ppb 30-225 ppb

117 ( 2 15) ppb 50-220 ppb

TABLE 111. Percent Neutralization per ppb Laryngeal NH3

d, = 0.5 pm dp = 1.0 pm

Entry via 0.28 0.06 mouth (0.08) (0.06)

Entry via 0.19 - nose (0.21)

Confidence limits of 95% are in parentheses.

452 Larsonetal

residence time did not vary significantly between dogs, this value reflects the average rate of reaction in the airways. As expected, for entry via the nose the value is greater for the 0.5-pm particles. Diffusion theory predicts a rate for 1.0-pm particles that is approximately one fourth the rate or 0.5-pm particles, a result consistent with our ob- servations. For 0.5-pm particles, the extent of neutralization of laryngeal NH, is larger when the particles enter via the mouth than when they enter via the nose. This result implies that the particles passing through the nose received a lower dose of NH3 than those passing through the mouth. This is consistent with our observation (not reported) that the NH3 levels in the nose of the dog are lower than those in the mouth.

SUMMARY

In conclusion, we have shown that H2S04 particles can undergo significant neu- tralization during inhalation via the nose or mouth by the time the particles pass the larynx. The extent of neutralization depends both on NH3 concentration and on particle size, with 0.5-pm particles being neutralized about four times more rapidly than 1 .O-pm particles. Our results suggest that smaller particles are not necessarily more imtating than larger ones and that nose breathing is not necessarily more protective than mouth breathing. Finally, we have demonstrated a relatively simple method for studying these phenomena in respiratory airways.

ACKNOWLEDGMENTS

This research was supported by the National Institute of Environmental Health Sciences (grant #5P01ES01478)03 and by the U.S. Environmental Protection Agency (grant #R8053780 10)

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