WEARING your heart on your sleeve can elicit sympathetic smiles, but proclaiming--on your chest--sentiments about the state of the environment might rally cries to action. You can do just that with one of the T-shirts commissioned by the American Institute of Graphic Arts of Los Angeles in an effort to promote both the organization and college-student artists. An AIGA-sponsored contest produced three winning designs that decry the results of acid rain, pollution, the destruction of the rain forests and the depletion of the ozone layer. The 100% cotton Ts come in sizes large and extra-large and sell for about $18. AIGA T-shirts are available directly from the AIGA, (213) 653-1724; or at New Stone Age, 8407 W. 3rd St., Los Angeles, (213) 658-5969.