In talking about exposure to drugs, supplements, environmental contaminants, etc., it’s important not only to consider the dosage/concentration and duration of exposure but also how the substance got into a person’s body — the route of exposure (or exposure pathway). The most common routes of exposure from the environment are ingestion and inhalation, followed by dermal (or topical, including use of personal care products). In drug exposures, ingestion and injection are most common, though “injection” is a broad term also encompassing different routes of exposure. (Another type is implantation, and “occupational” can include multiple routes at once.) The same substance can have different effects depending on its route of exposure.
For example, people ingest formaldehyde every time they eat most fruits, vegetables and legumes, and formaldehyde is a common ingredient in injected drugs and vaccines in small amounts that don’t cause harm. Indeed, they’re typically less than the formaldehyde that the body itself produces during metabolism. Inhalation of formaldehyde, however, if at sufficient doses and/or durations, can cause cancer. Formaldehyde has never been demonstrated as carcinogenic when ingested or injected in tiny amounts.