Ozone Challenge:
Physical Prerequisites to Comparable Results
During ozone challenge procedures, test persons are breathing air which contains a small but precisely controlled amount of ozone. The exspired breath flow is measured with a pneumotachometer which measures and calculates the minute volume in l/min. The ozone content is defined and measured as parts per billion, volume of ozone by volume of air/ozone mixture (ppbv). Ozone content is held constant during a single challenge procedure.
The chemical and biological effects of an ozone challenge procedure are quite probably rising with rising total number of ozone molecules (i.e. total mass of ozone) inspired by the test person during the ozone challenge period. To make the results of different experiments comparable it is necessary to measure the total mass of ozone inspired during the procedure in order to compare ozone challenge procedures only with known total mass of ozone inspired by the test persons, or even with the same total mass of ozone.
Let us have a look on how to measure and calculate the total ozone mass inspired during an ozone challenge period:
The ozone content (ppbv) does not change with changing temperature and pressure of the inspired air/ozone mixture. The higher the density of the mixture is, the higher is the density of the ozone, too. Higher density of the mixture actually means: more ozone in the same volume of the mixture.
To determine the mass of ozone inspired by a test person it is not sufficient to measure the volume of the inspired mixture. In a given volume the mass of ozone depends not only on the ozone content but it also depends on the temperature and the pressure.
The influence of temperature is not significant as long as we are speaking about breathing at room temperature between about 20ºC and 30ºC. The variation in density in this temperature range is only about 3%.
Much more important is the pressure. Barometric pressure at a place on earth can change statistically by about -10% to +7% from the average. Fluctuations usually are lower, but still are significant. The average barometric pressure decreases with increasing height of the place. A rule of thumb: The average barometric pressure decreases by 10% per 1000 m of height above sea level.
The density of the air at constant temperature is inversely proportional to the pressure. This actually means that a given volume contains 10% less mass of air when the pressure is 10% lower. Considering the statistic fluctuations of the barometric pressure, and the changes in average pressure with height, one may readily have a barometric pressure difference of 10% between e.g. Hamburg and Munich. Between Los Angeles and Mexico City the difference can be more than 25%.
Two different ozone challenge procedures performed at two different places, but at the same minute volume (l/min), and with the same ozone content (ppbv) actually can differ concerning the inspired ozone mass by 10% (Hamburg/Munich) or even by 25% (Los Angeles/Mexico City).
This is the reason why professional ozone challenge demands measurement of the inspired ozonated air with a flow sensor which measures air mass flow rate in Nl/min instead of the air volume flow rate in l/min. One Nl (normal litre) of a gas is the mass (!) which occupies the volume of 1 litre at normal conditions (0ºC and 1 atm). The air mass flow rate multiplied with the ozone content gives the inspired ozone mass flow rate (ozone dose rate).
Test persons never breath with constant minute volume. To be comparable with other experiments an ozone challenge procedure should expose the test person to a predetermined total mass of ozone (ozone dose). This is the reason why the air mass flow rate should be totalized during the procedure until a certain predetermined total mass of inspired air has been reached. Since the ozone content in the inspired air is held constant during the procedure, this total mass of inspired air contains a certain mass of ozone (a certain ozone dose). When the predetermined ozone dose is reached the ozone challenge procedure now should be stopped. Stopping an ozone challenge procedure only after a predetermined time interval may lead to ozone doses significantly changing from procedure to procedure.
Test persons breath through 2-way-non-rebreathing valves ("Y-valves") in order to separate the inspired air/ozone mixture from the exspired air. These valves have an internal dead space. The dead space is filled with exspired air during exspiration. During inspiration this air (which is supposed to be free of ozone, more or less) mixes with the inspired air/ozone mixture. An example: Let the tidal volume be 1 litre, and the dead space volume of the valve be 100 ml. The test person now inspires 900 ml of fresh air/ozone mixture plus 100 ml of exspired air. Dilution of the ozonated air actually inspired is the inevitable consequence. The ozone content now is 10% lower than might be assumed. If the tidal volume would be increased to e.g. 2 litres, the degree of dilution would change from 10% to 5%. The degree of dilution actually changes with changing tidal volume the more, the higher the dead space volume is. We have seen a group using a 2-way-non-rebreathing valve with a dead space volume of 180 ml. On the other hand, professional valves designed for scientific ergospirometry offer a dead space volume of only 30 ml. Such a low dead space of course is possible only when the pneumotachometer (flow meter) is not installed in the 2-way-non-rebreathing valve. Instead it should be mounted at the inlet of the inspiratory breath tube, or even inside the ozone delivery system. The flow meter now is no more subject to contamination by saliva. It does not need regular cleaning.
To close this short excursion on some physical aspects of ozone challenge, it should be mentioned that the effects of the inspired ozone actually have to be judged in light of how much ozone is really "consumed" by the test person. This actually cannot be more than the difference between the mass of ozone inspired and the mass of ozone exspired. Thus the ozone content in the exspired air should be measured, too, as long as it has not yet been proven that exspiratory ozone content is negligible. With our new OZONE EXPOSURE SYSTEM OES 1.1 under development we will try to find out how much ozone is exspired, and which variations in exspired ozone have to be considered.
Dr.-Ing. Franz Wallner
July 2005