Tariff
A tariff is called an optimal tariff if it's set to maximize the welfare of the country imposing the tariff. It is a tariff derived by the intersection between the "trade indifference curve" of that country and the offer curve of another country. In this case, the welfare of the other country grows worse simultaneously, thus the policy is a kind of "beggar thy neighbor policy". If the offer curve of the other country is a line through the origin point, the original country is in the "condition of a small country", so any tariff worsens the welfare of the original country.
Eco-Tariffs
One of the major issues that are raised when discussing environmental tariffs, is the issue of a reduction in trade. The argument raised is that tariffs reduce trade and may not actually be targeting the actual source of the pollution. They argue that pollution is not just as a result of imported goods but a large part of pollution suffered occurs within the borders of a country, therefore trade would merely harming trade without actually addressing the root cause effectively.