CORRESPONDENCE
Short-Term Effects of Air Pollution on Mortality in New York City SIR: We have read with great interest the article by T. A. Hodgson (ENVIRON.SCI. & TECHNOL. 4, 589-597, 1970). Although not professing t o be statisticians and, therefore, admitting t o no competence to judge the validity of the statistical methods used, we are concerned about certain concepts expressed in the article and believe they need careful analysis and evaluation. These may be listed arld are discussed as follows: ONE. I t seems to us that the opening sentence contains a contradiction. If “very little is known of the qualitative or quantitative nature of the relationship between air pollution and health,” then how can “considerable evidence” be present t o “support the belief. . .that air pollution affects health”? Evidence, we presume, means data, and data are either qualitative or quantitative knowledge. Is there another type of knowledge of which we are unaware ? Two. On page 590, Hodgson indicates that particulates were measured in coh units and sulfur dioxide in ppm. Later, on page 596 he indicates that he has used increments of 1 unit in the average coh units and 0.5 unit in the average concentration of sulfur dioxide. Presumably these are monthly averages. We have operated a n air monitoring station for over two years and have never observed a variation in monthly averages for smokeshade of 1 coh unit and of SO2 of 0.5 ppm. Hodgson stipulates day-to-day variations of as much as 5 coh units and 0.5 ppm of SO?. Monthly averages never have even approximated such wide swings. Such a change in monthly averages would represent a whopping change in air pollution levels. As a matter of fact, our monthly averages for smokeshade are about 0.5 t o 1.0 coh unit and for sulfur dioxide 0.04 to 0.08 ppm. If changes of monthly averages of the magnitude indicated by Hodgson did occur, then we would not be surprised at all if increased mortality occurred, since this would necessitate several days each month when coh units would exceed 9 or 10 and SO2 levels 2 or 3 ppm. Such levels of pollutants we would classify as “episodes.” THREE.On page 594, Hodgson states “the implication of a linear relation between mortality and the environmental factors is that a unit change in the level of air pollution in-
SIR: The criticism of Eckardt, Scala, and Brief is replete with misconception, misinterpretation, and factual error. Part of the difficulty they encountered can be attributed to their admitted lack of statistical knowledge. The bulk, however, is due to a careless reading of my paper and misstatement of certain facts. A thorough rebuttal requires a lengthy reply. However, I shall endeavor t o be as brief as possible and respond t o the five points made by Eckardt, et al., in the same order as in their letter: 548 Environmental Science & Technology
duces as great an increase in mortality a t low levels of pollution as at higher levels.” T o the toxicologist and pharmacologist, this is contrary t o any dose-response curve ever constructed unless the response is being measured in a very restricted area of the dose range. From what has been said above in paragraph 2 , this does not appear to be the case with Hodgson’s data on dose. Any toxicologist or pharmacologist knows that the dose-response curve is not linear but S-shaped. This means that for a unit increase at low doses the response is small, becoming large at intermediate doses and again tapering off t o be small at high doses. Only if one were comparing responses soley in the intermediate area of dose, or comparing responses in the low and high areas with each other, would that linearity exist. To propose linearity in the dose-response curve would be t o negate the thousands of toxicological and pharmacological experiments that have been conducted over the years which show that dose-respose curves are nonlinear. That Hodgson’s data imply linearity suggests some basic defect in the data which he was manipulating statistically. FOUR.The validity of using coh units as a measure of particulate concentration is open to serious question. They measure only the blackness of particles, not their concentration. Particulate concentrations are usually measured in units of micrograms per cubic meter. FIVE. That people 64 and under in age cannot be on the brink of death is an erroneous concept. Those ill at ages 64 or under are dying regularly regardless of air pollution levels, and since there are more people age 64 and under than over 65, one would expect more daily deaths among the 64 and under age group than among the 65 and over age group. R. E. Eckardt R. A. Scala R. S. Brief Esso Research and Engineering Co. Medical Research Dicision Linden, N . J. 07036
ONE. An exact quotation of my opening sentence is “Although there is considerable evidence t o support the belief, now widely held, that air pollution affects human health, very little is known of the qualitative or quantitative nature of the relationship between air pollution and health.” Referring t o the 1966 unabridged edition of the “Random House Dictionary” one will find that considerable means “worthy of respect, attention, etc.; important” (as well as “rather large or great, as in size, distance, extent, etc.”); support means
