Scientists can measure tracheobronchial clearance by having a subject inhale a radioactive aerosol and then later metering the radiation level in the lungs. In one experiment (22) seven pairs of monozygotic twins inhaled aerosols of radioactive Teflon particles. One member of each twin pair lived in a rural area; the other was a city dweller. The objective of the study was to see whether the data would support the popular contention that rural environments are more conducive to respiratory health than are urban environments. The table adjacent gives the percentages of radioactivity retained in the lungs of each subject 1 hr after the initial inhalation. Set up and carry out the appropriate hypothesis test. Let α = .05.

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Radioactive Retention (%) Twin Pair Rural, x Urban, y - 2 3 4 5 6 7 10.1 51.8 33.5 32.8 69.0 38.9 54.6 28.1 36.2 40.7 38.8 71.0 47.0 57.0