You can try to create your Emergency Response Plan on your own. However, if you look at the compliance issues, you will start to appreciate the complexity and liabilities of this undertaking.
An all-hazards plan is now the best practice and is required if you subscribe to NFPA 1600 the national standard recognized by the US Congress. All-hazards is also necessary because your lawyer will not want to send you into your deposition or trial testimony to say, "We didn’t think of that," or "We didn’t want to spend time and money planning and training for all that."
There are competent expert consultants who can help.
911 Consulting is one of those experts.