If smell can define a place, New York in the summertime is not exactly a meadow of flowers. In the war between a sensitive nose and the city streets, the streets have the upper hand, assailing the nose with the odors of urine, decomposing garbage and clammy armpits. Mouth breathing is not only acceptable, but often necessary.

So when several people armed with pens and paper tipped toward a moist sewer grate at the corner of Fifth and Berry Streets in Williamsburg, Brooklyn, and took a delicate sniff on Wednesday evening, a Buildings Department phrase, “conditions perilous to human life,” seemed apropos. They paused there, nostrils twitching, eyebrows furrowed.

“It’s a salsa-y smell,” offered one, Alexandra Horowitz. The group hovered some more, undecided.

“Garlic!” exclaimed Sam Vale, 35, to general delight. It was unmistakable, a stench so pungent that even the most congested of his companions took an involuntary step back.

“Good God,” one muttered.

For the participants in the Smellwalk, the worst was yet to come. Meandering down a city block pocked with olfactory bombs — artisanally roasted coffee; Acqua di Gio cologne, emanating from a European tourist; blended wheatgrass — they were charged with stopping every so often to smell, as it were, the roses of Bedford Avenue.