This is partly a question of science, but it is really more of a question of what government should do. It is easy to answer the scientific aspect of the question, but the social science aspect is much harder. That part of the question is really a matter of personal political views.
There is no question that lawn mowers that run on gas are bad for the environment. The link below tells us that a government agency in the state of Texas found that lawnmowers actually emit four times more pollutants than cars do for each hour of use. Lawn mowers are clearly a source of pollution and catalytic converters would ameliorate the problem. Therefore, scientifically speaking, it would be better to mandate that lawn mowers include catalytic converters.
The harder question, though, is the political question. Here, we enter the realm of opinion. What we have to decide is whether the problem is important enough to warrant government interference. We have to decide if the benefit we get is greater than the cost we incur. What cost do we incur? We incur a small economic cost because we have to pay slightly more for a lawn mower with a catalytic converter. Most people will not, however, see this as much of a burden. They may not even notice the increase in price. The more significant price, in many people’s eyes, is the price we pay in terms of liberty.
Many people, mostly conservatives, believe that government regulations reduce our liberty. They believe that we have the right to produce and to buy essentially whatever we want. The government should only step in and tell us what to do when it is very important. From this point of view, if people want catalytic converters on their lawn mowers, they should let the lawn mower companies know this. If there is demand, the companies will create such mowers. If enough people want the cleaner mowers, the companies will stop producing dirty mowers. However, people should have the right to buy the dirtier mowers if they want and if companies are willing to sell them. In this way, people’s economic rights are honored. If the government simply tells us that we have to have catalytic converters on our mowers, we lose some of our freedom. Of course, we can argue that lawn mowers are not a major part of our freedom, but conservatives would argue that every time we give up some of our freedom, we make it easier for government to take away more important rights because we get used to letting the government tell us what to do.
Liberals do not agree. They would say that the right to buy whatever kind of lawn mower you want is a very minor right. They would say that the loss of that right is more than made up for by the improvement in environmental quality. They would not have any doubt that requiring catalytic converters is a good idea.
So, the question is which of these arguments you find more persuasive. Is it dangerous for us to let the government tell us what to do or is environmental quality worth some minor reduction in our right to do whatever we want?