Mishnayos Taharos Perek 3 Mishnah 4

If an egg's bulk of food was left in the sun and it was lessened, and so also in the case of an olive's bulk of corpse, an olive's bulk of carrion, a lentil's bulk of a sheretz, an olive's bulk of piggul, an olive's bulk of notar, or an olive's bulk of forbidden fat they become clean; Nor is one liable on account of these for transgressing the law of piggul, notar or forbidden fat. If they were then left out in the rain and they swelled, they become unclean and guilt is incurred on account of them for transgressing the law of piggul, notar or forbidden fat.

כַּבֵּיצָה אֳכָלִין שֶׁהִנִּיחָן בַּחַמָּה וְנִתְמַעֲטוּ, וְכֵן כַּזַּיִת מִן הַמֵּת, וְכַזַּיִת מִן הַנְּבֵלָה, וְכָעֲדָשָׁה מִן הַשֶּׁרֶץ, כַּזַּיִת פִּגּוּל, כַּזַּיִת נוֹתָר, כַּזַּיִת חֵלֶב, הֲרֵי אֵלּוּ טְהוֹרִים, וְאֵין חַיָּבִין עֲלֵיהֶן מִשּׁוּם פִּגּוּל, נוֹתָר וְטָמֵא. הִנִּיחָן בַּגְּשָׁמִים וְנִתְפְּחוּ, טְמֵאִין, וְחַיָּבִין עֲלֵיהֶם מִשּׁוּם פִּגּוּל, נוֹתָר וְטָמֵא: